libtest: rename `BenchHarness` to `Bencher`
Closes #12640
Add more type signatures to the docs; tweak a few of them.

Someone reading the docs won't know what the types of various things
are, so this adds them in a few meaningful places to help with
comprehension.

cc #13423.
auto merge of #13236 : liigo/rust/rename-benchharness, r=huonw

Closes #12640

based on PR #13030, rebased, and passed all tests.
auto merge of #13458 : huonw/rust/doc-signatures, r=alexcrichton

Add more type signatures to the docs; tweak a few of them.

Someone reading the docs won't know what the types of various things
are, so this adds them in a few meaningful places to help with
comprehension.

cc #13423.