Define cfg(rtopt) when optimizing. Turn off runtime sanity checks

Naturally, and sadly, turning off sanity checks in the runtime is
a huge performance win. The particular test I'm running goes from
~1.5 s to ~1.3s.

This also converts various TLS code paths to do unsafe but more
efficient things when sanity is off.

Sanity checks are turned *on* when not optimizing, or when cfg
includes `rtdebug` or `rtassert`.
std: Convert some assert!s to rtassert!
std: More TLS micro-optimization
std: Convert the runtime TLS key to a Rust global to avoid FFI