This commit is a small refactoring of `wasmtime_runtime::Trap` and
various internals. The `Trap` structure is now a reason plus backtrace,
and the old `Trap` enum is mostly in `TrapReason` now. Additionally all
`Trap`-returning methods of `wasmtime_runtime` are changed to returning
a `TrapCode` to indicate that they never capture a backtrace. Finally
the `UnwindReason` internally now no longer duplicates the trap reasons,
instead only having two variants of "panic" and "trap".
The motivation for this commit is mostly just cleaning up trap internals
and removing the need for methods like
`wasmtime_runtime::Trap::insert_backtrace` to leave it only happening at
the `wasmtime` layer.
77e06213b7