Consolidate local_data implementations, and cleanup

This moves all local_data stuff into the `local_data` module and only that
module alone. It also removes a fair amount of "super-unsafe" code in favor of
just vanilla code generated by the compiler at the same time.

Closes #8113
auto merge of #8447 : alexcrichton/rust/local-data-merge, r=brson

This moves all local_data stuff into the `local_data` module and only that
module alone. It also removes a fair amount of "super-unsafe" code in favor of
just vanilla code generated by the compiler at the same time.

Closes #8113