std::result: Remove function `map_opt`

This function has never had any users in the tree
Don't die in try_unsafe_borrow if tls isn't ready

If there's no TLS key just yet, then there's nothing to unsafely borrow, so
continue returning None. This prevents causing the runtime to abort itself when
logging before the runtime is fully initialized.

Closes #9487
Document std::ops.

Added an overview with a 'real' example, as well as toy implementations of all of
the traits.

Closes #9356.