Don't build docs/tests into separate dirs

This commit changes the hash of Profile to only take into account
flags/variables that affect the actual output file itself (as opposed to its
location), and then changes cargo {test, build, doc} to all use the same
directory of output (in order to share deps).

This will cause a `cargo build` to remove all of the tests generated by `cargo
test`, but it speeds up the cycle of `cargo test` followed by a `cargo build` by
not needing to rebuild all dependencies.

Additionally, `cargo bench` now shares the same directory as
`cargo build --release` for the same reasons as above.

Closes #348