std: migrate path::unix to using Vec internally.
std: migrate path::windows to using StrBuf internally.

Same representation change performed with path::unix.

This also implements BytesContainer for StrBuf & adds an (unsafe) method
for viewing & mutating the raw byte vector of a StrBuf.
std: update & de-~[] path's tests.
auto merge of #13481 : huonw/rust/devec-path, r=alexcrichton

Remove the use of ~[] from Path's internals.