extra: Put the nail in the coffin, delete libextra

This commit shreds all remnants of libextra from the compiler and standard
distribution. Two modules, c_vec/tempfile, were moved into libstd after some
cleanup, and the other modules were moved to separate crates as seen fit.

Closes #8784
Closes #12413
Closes #12576
auto merge of #12896 : alexcrichton/rust/goodbye-extra, r=brson

This commit shreds all remnants of libextra from the compiler and standard
distribution. Two modules, c_vec/tempfile, were moved into libstd after some
cleanup, and the other modules were moved to separate crates as seen fit.

Closes #8784
Closes #12413
Closes #12576