Porting std.typecons to Phobos 3

[Adam Wilson]

## Porting `std.typecons` to Phobos 3

While JMD continues his work on Ranges V3 he mentioned that it 
would help him out if we could get started porting `std.typecons` 
to Phobos 3. Therefore, I'd like to open this module up to the 
community to see what everyone thinks of the plan we've developed.

The first thing I'd like to consider is renaming to 
`phobos.sys.types`. It is difficult to name `std.typecons` 
because it is a grab-bag of type-related tools. We can continue 
with `typecons` if desired, however, I would note that "cons" is 
not a standard abbreviation for "constructor", that'd be "ctor".

Second, JMD proposed that we re-organize this module into a 
package due to it's grab-bag nature. While I want to stress that 
we won't be doing this with every module Walter has asked that we 
evaluate modules for reasonable splits to make the files smaller 
and easier to read. Given the nature of `std.typecons` we think 
that this module is an excellent candidate. My proposal is to 
break this module up by the categories in the document with the 
exception of `Nullable` which will get it's own file module due 
to it's size.

Speaking of `Nullable`, after a rather embarrassing situation 
with a Phobos 2 PR on `Nullable` JMD and I agreed that it should 
be renamed as it does not actually behave like a nullable but 
instead behaves as an "Optional" type. We considered `Optional`, 
`Maybe`, and settled on `Option` as that appears to the more 
popular naming choice in other languages and is almost as short 
as `Maybe`. If you disagree I'd love to hear your case.

The last question is how we want to deal with existing bugs in V2 
as we port the code to V3. My view is that it would be prudent to 
fix any bugs that have been opened as we go in both V2 and V3. 
Currently I've found a number of bugs against `Nullable` and 
`Tuple`, the two types I'd like to start with.

- First, some of these bugs are likely already fixed and just 
didn't get cleared by the bot for whatever reason, but I don't 
have the domain knowledge to know which, if any, already have 
been. If somebody could flag those I'll get them closed.
- Second, because some of these fixes will involve breaking 
changes, we need to decide which bugs we want to fix in both V2 
and V3 and which we want to fix only in V3. If you could reply 
with "Both" or "V3" for fixes that'd be most helpful.
- Lastly, I found these via simple search through the bug list, 
if they don't apply, or if I am missing any, please let me know.

### Nullable Bug List:
[9637 - std.typecons.nullable and nullableRef 
functions](https://issues.dlang.org/show_bug.cgi?id=9637) (It 
looks to me like these exist, do the existing methods satisfy the 
issue?)
[12323 - std.typecons.Nullable of a fixed-size 
array](https://issues.dlang.org/show_bug.cgi?id=12323)
[13092 - std.algorithm.cartesianProduct of Nullable Tuple of 
strings array too](https://issues.dlang.org/show_bug.cgi?id=13092)
[17615 - Deprecate using reference types for 
Nullable](https://issues.dlang.org/show_bug.cgi?id=17615)
[21420 - Nullable.get default parameter is not 
lazy](https://issues.dlang.org/show_bug.cgi?id=21420)
[22279 - alias this in Nullable(T, T nullValue) causes Error in 
writeln](https://issues.dlang.org/show_bug.cgi?id=22279)
[24270 - Pointer to Nullable of recursive 
struct](https://issues.dlang.org/show_bug.cgi?id=24270)
[24233 - T[].canFind(Nullable!T()) returns true when it used to 
return false](https://issues.dlang.org/show_bug.cgi?id=24233)

### Tuple Bug List:
[8494 - Return value for 
Tuple.opAssign](https://issues.dlang.org/show_bug.cgi?id=8494)
[9593 - Optional type-free printing of a 
std.typecons.Tuple](https://issues.dlang.org/show_bug.cgi?id=9593)
[12743 - tuple printing of alias 
names](https://issues.dlang.org/show_bug.cgi?id=12743)
[13104 - 
std.typecons.tupleOp](https://issues.dlang.org/show_bug.cgi?id=13104)
[15740 - Indexing a std.typecons.Tuple with names doesn't work in 
ctfe](https://issues.dlang.org/show_bug.cgi?id=15740)
[15905 - Tuple Op Assignment Overload 
incorrect?](https://issues.dlang.org/show_bug.cgi?id=15905)
[18426 - Conversion from Tuple with named fields to Tuple without 
named fields should work during 
compilation](https://issues.dlang.org/show_bug.cgi?id=18426)
[20031 - Tuple.toString does not work with mutable 
toString](https://issues.dlang.org/show_bug.cgi?id=20031)
[20228 - simple toString for 
Tuple!()s](https://issues.dlang.org/show_bug.cgi?id=20228)
[20966 - std.typecons.Tuple cannot handle non-copyable 
types](https://issues.dlang.org/show_bug.cgi?id=20966)
[21011 - Variant and tuples by 
index](https://issues.dlang.org/show_bug.cgi?id=21011)
[21401 - Writelning a tuple ignores member's 
toString.](https://issues.dlang.org/show_bug.cgi?id=21401)
[21429 - Cannot sort large tuple arrays at compile 
time](https://issues.dlang.org/show_bug.cgi?id=21429)
[21888 - sorting medium sized tuples at compile time not 
possible](https://issues.dlang.org/show_bug.cgi?id=21888)
[24138 - [REG 2.103] MemberFunctionsTuple does not see final 
functions](https://issues.dlang.org/show_bug.cgi?id=24138)
[24285 - Cannot swap a 
std.typecons.Tuple](https://issues.dlang.org/show_bug.cgi?id=24285) ([Open PR for this Issue](https://github.com/dlang/phobos/pull/8864))