rust_library should produce non-rlib crates with correct file extensions

[davidzchen]

Currently, the rust_library rule only outputs rlibs regardless of the value of crate_type. In order to correctly support all the different crate types, `rust_library should produce the crates with the correct file extensions.

Proposal

• Make the rust_library rule internal by renaming it to _rust_library
• Add a new outs attribute to the _rust_library rule to support custom outputs
• Add a rust_library macro that wraps the _rust_library rule. Based on the value of crate_type, it adds a _rust_library rule with outs set with the name of the output library with the correct file extensions according to the Rust documentation

[davidzchen]

+cc @dslomov @laurentlb @acmcarther @mattmoor @damienmg

[damienmg]

SGTM

[acmcarther]

This was fixed in:
#38

... which produces outputs with the right file extension for the toolchain and crate_type.