This commit fixes a few issues around managing the thread-local state of
a wasmtime thread. We intentionally only have a singular TLS variable in
the whole world, and the problem is that when stack-switching off an
async thread we were not restoring the previous TLS state. This is
necessary in two cases:
* Futures aren't guaranteed to be polled/completed in a stack-like
fashion. If a poll sees that a future isn't ready then we may resume
execution in a previous wasm context that ends up needing the TLS
information.
* Futures can also cross threads (when the whole store crosses threads)
and we need to save/restore TLS state from the thread we're coming
from and the thread that we're going to.
The stack switching issue necessitates some more glue around suspension
and resumption of a stack to ensure we save/restore the TLS state on
both sides. The thread issue, however, also necessitates that we use
`#[inline(never)]` on TLS access functions and never have TLS borrows
live across a function which could result in running arbitrary code (as
was the case for the `tls::set` function.
918c012d00