implement 'use' for rustc

[graydon]

Rustc can't 'use' crates yet. In fact it can't even write out crate metadata. It has to learn how.

Tasks here involve:

• Make up a serial encoding of crate metadata that is efficient to incrementally parse and
  seek around inside of, but also stable and simple enough that we're not likely to break it
  all the time as we hack on rustc.
• Compose a metadata blob of this sort and write it out as a special .note.rust section via
  LLVM. Not sure exactly what support it has for making up our own section names. Try
  your best.
• Mangle the symbols inside a given crate using the mangling scheme outlined in this
  mailing list post: https://mail.mozilla.org/pipermail/rust-dev/2010-December/000164.html
• When you 'use' a crate, scan for the .note.rust section (might need to port the objfile
  sniffer from rustboot, see for example boot/be/elf.ml 'sniff' and friends) and read its
  contents into the compilation-crate namespace. Lazily load additional components of
  the 'use'd crate namespace as references and imports demand.

[pcwalton]

"Not sure exactly what support it has for making up our own section names. Try your best."

LLVM supports this, and the stub code now in the tree does it.

[marijnh]

Some related design in https://github.com/graydon/rust/wiki/Crate-format-rfc

[pcwalton]

Marking as fixed; this basically works now.