Support 'bail-out' from native code to signal debug/error-states/branches.

[twitchyliquid64]

Just thinking aloud here, would appreciate any thoughts/feedback.

It would be good for native code to be able to return early to signal different states back to the interpreter.

• Debug: Stack/local handling code can do bounds checks and return a value if bounds are every violated. We dont expect this to happen for verified modules, but it would be good to have this as a build flag like we have for the interpreter (to be sure).
• (in the future) Branching: we can compute/store an 'outgoing branch table' for each native code sequence, and the native section can return back a specific code to signal a branch to another function, or even stuff like function calls.

Keeping things simple, I think we should implement this signalling through a int64 value on the stack, and by the native code returning. The int64 value can be packed something like this:

04 Bits: Return code (0 = no error, 1 = bounds check failed, 2 = unreachable, 3 = function call, 4 = branch, 5 = fatal error, remaining codes reserved
04 Bits: Reserved for future flags.
24 Bits: Index of the WASM instruction where the bail-out happened (ie: index into `BytecodeMetadata.Instructions`)
32 Bits: Jump target / Branch index / Code-specific-data.