Get dynamic loading working

[graydon]

For syntax extensions (and possibly semantic extensions) to load in from external crates, we need to dynamically load them. Currently the library-scanning machinery is a bit too weak to handle this, and there's no os-level interface in the standard library for fetching symbols from a loaded library. Implement all this.

[graydon]

Pushing this out to post-0.1 release

[brson]

Doing this in a safe way is likely blocked on #2213

[pcwalton]

I don't believe this is backwards incompatible, renominating.

[huonw]

cc #6735 (a proof of concept).

[graydon]

accepted for feature-complete milestone

[emberian]

#7027 is a solid step forward on this.

[thestinger]

This is implemented, and there are tests for it in src/libstd/unstable/dynamic_lib.rs.

[graydon]

The caveat I have here is that the interface isn't typesafe. Only works with symbols with mangling turned off. Guess the bug didn't mention that but imo it's important.

[catamorphism]

High, but not 1.0