* Introduce Trap struct.
* get_local can't fail.
* Add MemoryOutOfBounds trap.
* from_little_endian use slice instead of vec.
* MemoryAccessOutOfBounds for mem get and set.
* from_little_endian conversion can't fail.
* call_indirect traps.
* DivisionByZero and InvalidConversionToInt
* Use traps in value to convey an error
* select: int condition on stack top
* if: int condition on stack top
* Assert pops.
* Another protions of assert pops
* Introduce ValueStack
Also, hide FunctionContext and remove some stale code
* Traps in execution
* Make it compile.
* Check args before invoke.
* Document RuntimeArgs.
* Update host.rs
* Add rustdoc for Trap.
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