Add a signals-based-traps Cargo compile-time feature

[alexcrichton]

This PR adds a new signals-based-traps Cargo compile-time feature which mirrors the Config::signals_based_traps runtime configuration. Disabling this Cargo feature enables reducing Wasmtime's platform requirements to remove the need for virtual memory and trap handlers for example. That means that the underlying platform requirements are just setjmp/longjmp and a single pointer of thread-local storage. This should make it significantly easier to port Wasmtime to platforms that may not have virtual memory in the future.

I'll note that one of the main purposes of this PR isn't quite realized just yet which is to run Pulley on embedded systems. Pulley isn't finished, however, so this is more setting the stage for that to be possible in the future than enabling it right this red-hot-second.

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[pchickey]

Pretty cool!

[alexcrichton]

I've pushed some extra refactoring of MmapVec as well to handle the internals a bit better given today's requirements.