Auto merge of #1558 - alexcrichton:less-hashes, r=brson

The root crate often has artifacts which are later intended for distribution of
some form, so adding a hash will just make predicting the file name difficult.
The hash also isn't necessary as it's guaranteed to not conflict with other
files (no other dependencies are in the same directory) and all other libraries
have metadata so symbols will not conflict.

Closes #1484

diff --cc src/cargo/ops/cargo_rustc/mod.rs
Simple merge

diff --cc tests/test_cargo_compile.rs
Simple merge

diff --cc tests/test_cargo_compile_custom_build.rs
Simple merge