libsyntax: Remove the obsolete ability to parse from substrings.

This was used by the quasiquoter.
Merge Bitwise and BitCount traits and remove from prelude, along with Bounded

One less trait in std::num, and three less exported in the prelude.
auto merge of #11548 : bjz/rust/bitwise, r=alexcrichton

One less trait in `std::num` and three less exported in the prelude.

cc. #10387
auto merge of #11575 : pcwalton/rust/parse-substrs, r=alexcrichton

This was used by the quasiquoter.

r? @alexcrichton