Make sure that wasmtime gets all the traps it needs

[syntactically]

Currently, we only properly call the trap handler given to wasmtime_init_traps() when an undefined instruction is executed. #113 should ensure that trapping accesses to virtual memory are also handed to the trap handler, but there are probably some other exceptions (e.g. floating point) which need to be handed to wasmtime. We should figure out what all of them are and make sure they are properly sent to wasmtime.