Move everything syntax-related to syntax/, break deps on rest of compiler

src/comp/syntax is currently just a sub-module of rustc, but it will,
in the near future, be its own crate. This includes:

 - The AST data structure
 - The parser
 - The pretty-printer
 - Visit, walk, and fold
 - The syntax extension system
 - Some utility stuff that should be in the stdlib*

*) Stdlib extensions currently require a snapshot before they can be
   used, and the win build is very broken right now. This is temporary
   and will be cleaned up when one of those problems goes away.

A lot of code was moved by this patch, mostly towards a more organized
layout. Some package paths did get longer, and I guess the new layout
will take some getting used to. Sorry about that!

Please try not to re-introduce any dependencies in syntax/ on any of
the other src/comp/ subdirs.

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