compiletest: Remove all uses of `~str` from `compiletest`
librustuv: Remove all uses of `~str` from `librustuv`
libfmt_macros: Remove all uses of `~str` from `libfmt_macros`
libserialize: Remove all uses of `~str` from `libserialize`.

Had to make `struct Tm` in `libtime` not serializable for now.
libgetopts: Remove all uses of `~str` from `libgetopts`
doc: Remove all uses of `~str` from the documentation.