built-in array ptrEnd

[monarch_dodra]

I love D's concept of arrays (fat pointers).

However, one thing I've found it lacks is a (convenient) way to 
get the end ptr.

Phobos (and druntime) are riddled with "arr.ptr + arr.length". It 
is ugly and inconvenient, and makes something that should be easy 
to understand that much harder.

Then I thought: "std.array" defines all the functions required to 
enhance arrays. Why not just add a "ptrEnd" in there? So I did.

The rational is that now, we can write:

bool isDisjoint = a.ptrEnd <= b.ptr || b.ptrEnd <= a.ptr;

More elegant than:

bool isDisjoint = a.ptr + a.length <= b.ptr ||
                   b.ptr + b.length <= a.ptr;

Nothing revolutionary, but it *is* easier on the fingers when 
typing :D . Also, it *does* make a change in some bigger and more 
complicated cases.

Anyways, pull request:
https://github.com/D-Programming-Language/phobos/pull/798

I wanted to have some feedback, as this is introducing something 
new (as opposed to fixing something existing).

IMO, this should really be built-in, in particular, since, in my 
understanding, an array is internally represented by the ptr and 
ptrEnd pair anyways. If the compiler has access to it, it might 
as well communicate it (rather than us re-calculating it...)