Phobos begat madness

[Walter Bright]

In trying to get Phobos to compile with dip1000, I was faced with the following 
innocuous looking line:

   assert(equal(achr, arr), text(a.byCodepoint));

   https://github.com/dlang/phobos/blob/master/std/uni.d#L3763

and the compiler complained that `text(a.byCodepoint)` was a system call. I 
decided to accept the Mission Impossible of figuring out why it was inferred as 
@system.

I found myself getting lost in a maze of twisty template expansions, all looking 
slightly different:

   text
   begat textImpl
   begat to
   begat toImpl
   begat toStr
   begat formatValue
   begat formatValueImpl
   begat put
   begat doPut
   begat Appender.put
   begat Appender.put

These templates are all just wrappers around wrappers. They're laden with 
overloads, constraints and static ifs, so finding just what code is being 
compiled with what parameters is absurdly difficult. I can't begin to defend this.

Converting a range of empty/front/popFront to a string should be a simple, 
direct piece of code.

For example, simple conversion to a string should not need to detour through a 
complex formatting function.

(For those wondering about the genesis of 
https://github.com/dlang/dmd/pull/8384, trying to figure this out motivated it. 
Saved me lots of time, especially in finding which overload was being called.)

If someone wants to figure out how to build an uncomplicated, straightforward, 
efficient implementation of text() that we can be proud of, that would be a nice 
contribution.