Add doctests for keys() and values() of the BTreeMap collection.

Update keys() and values() for BTreeMap based on @Gankro's comments.

Assign keys and values to variables before doing assertion.
libcollections: remove unnecessary `as_slice()` calls
liballoc: remove unnecessary `as_slice()` calls
libcoretest: remove unnecessary `as_slice()` calls
libflate: remove unnecessary `as_slice()` calls
libfmt_macros: remove unnecessary `as_slice()` calls
libgetops: remove unnecessary `as_slice()` calls
libgraphviz: remove unnecessary `as_slice()` calls
liblog: remove unnecessary `as_slice` calls
libregex: remove unnecessary `as_slice` calls
libregex_macros: remove unnecessary `as_slice` calls
librustc: remove unnecessary `as_slice()` calls
librustc_back: remove unnecessary `as_slice()` calls
librustc_trans: remove unnecessary `as_slice` calls
librustdoc: remove unnecessary `as_slice()` calls
libserialize: remove unnecessary `as_slice()` calls
libstd: remove unnecessary `as_slice()` calls
libsyntax: remove unnecessary `as_slice()` calls
libterm: remove unnecessary `as_slice()` calls
libtest: remove unnecessary `as_slice()` calls