It's a constant, anyway.

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.

Just like it was done with items and exprs. Simplifies some code.

This reverts commit 6720ea76.

Closes #526

(Using the * operator.)

This makes tags more useful as nominal 'newtype' types, since you no
longer have to copy out their contents (or construct a cumbersome
boilerplate alt) to access them.

I could have gone with a scheme where you could dereference individual
arguments of an n-ary variant with ._0, ._1, etc, but opted not to,
since we plan to move to a system where all variants are unary (or, I
guess, nullary).

Some rather dodgy code was added to trans in the process. I'd love to
discuss it with someone who knows more about types during translation.