(Incidentally, what's the right way to test changes to the
pretty-printer?  There has to be a better way than what I did, which
was to log_err the results of expr_to_str as exprs passed through
trans and glance at the output.)

Once populated, this will allow us to load type info for types defined in
external crates referenced by other external crates.

the common case of wanting to link statically with the project's libraries
but dynamically with the system ones.

The encoding is very simple right now, just the crate name. Ultimately this
won't be enough for our versioning needs, but it should fill our immediate
need of being able to correlate encoded crate numbers to actual crates.

This involves pulling the type_abbrev cache out of crate_ctxt

The scope of external crate reading is beyond the resolve pass now, as it
builds up tables of information needed for several different purposes.

I intend for this to be the location for storing all the data retrieved by
creader, most of which is currently in the session.

That is, for example, import x::y::x, which defines a local x,
and thus wouldn't be able to find x::y anymore.

Closes issue #624

Closes issue #631

Removes ty::strip_boxes entirely, since unboxing is now more complicated
anyway.

trans::trans_lval will now autobind if the given expression was the
name of a generic functions. Those callees (trans_call and trans_bind)
that are interested in the generics information call trans_lval_gen
now.

Closes issue #639