A new trait to retrieve doc comments (if available).

[Mason McGill via Digitalmars-d]

**I'm fairly new to D, so let me know if this belongs in another 
thread.**

I'd like to contribute a new feature to the DMD front-end, and 
I'd appreciate some feedback on the design before I start on a 
pull request.

Feature:
========
`__traits(comment, symbol)` will evaluate to the doc-comment of 
`symbol`, if it is available, and "", otherwise. For DMD, this 
means it will provide comment information if the "-D" compiler 
option is used. Other implementations can choose to always 
evaluate it to "".

Use Cases:
==========
Here's my use case: I'm building an automatic wrapper generator 
for binding D to dynamic languages (mostly for scientific 
applications, at the moment). It's like SWIG, but more automated 
and narrower in scope.  Right now, I have two suboptimal options 
for supporting documentation comments:

1) Have users put their documentation in UDAs (instead of 
comments), and extract those.  This means departing from D style 
guidelines, and that DDOC doesn't work.

2) Dig comments out of DMD's JSON output.  This means users have 
to inform the wrapping tool of all of their D source files (not 
just a shared library), complicating build setups.  It also means 
DMD loads and parses each file twice.

Having doc-comments accessible at compile time would let me 
simplify the wrapping process for my users.

Other applications include metaprogramming (e.g. forwarding 
documentation from a template argument) and simplifying 
documentation generators. I'm sure having doc-comments accessible 
in Python made things like Sphinx and IPython easier to build.

Implementation:
===============
I'm not too familiar with DMD, but it seems like evaluating 
`__traits(comment, symbol)` would just require reading out the 
relevant `DSymbol`'s `comment` field.

Alternatives:
=============
Alternative names:
- `__traits(getComment, symbol)`
- `__traits(documentation, symbol)`
- `__traits(getDocumentation, symbol)`

Alternative behaviors:
- `__traits(comment, symbol)` could evaluate to `null` (rather 
than "") if there is no comment associated with `symbol`.

Thoughts?