cargo: add $HOME/.cargo/lib to rustc default search path

[graydon]

If it exists, the cargo lib dir should be added to the rustc default linking path. Figure out what counts as most-useful $HOME on windows.

[kud1ing]

Python's "os.path.expanduser(path)" does the following:

"On Windows, HOME and USERPROFILE will be used if set, otherwise a combination of HOMEPATH and HOMEDRIVE will be used. An initial ~user is handled by stripping the last directory component from the created user path derived above."

A comparable function could be added to "fs.rs"

[brson]

e746ed6 makes rustc aware of cargo's lib folder. Completely untested since I frankly can't get cargo to work.

[brson]

Fixxored