In 'rec(a = b with c)', if a exists in c then the expected type for b is the
type of c.

All visitors take a wb_ctxt now instead of some taking a fn_ctxt and some
taking an ignore flag.

(Possible now that a snapshot took place.)

Add a failure checkpoint after the typechecking pass. There are still many
fatal errors in typeck, but loosening up this one makes it easier to lean on
the compiler when making changes to types.

Issue #440.

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 means fewer mysterious error messages.

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).

This is important since we are going to be making functions noncopyable
soon, which means we'll be seeing a lot of boxed functions.
(*f)(...) is really just too heavyweight.

Doing the autodereferencing was a very little bit tricky since
trans_call works with an *lval* of the function whereas existing
autoderef code was not for lvals.