Idea:, VMs choose a spec level for run

[twitchyliquid64]

Problem statement: I want to use WebAssembly compliant with specification X.Y.

Suggestion:

1. Store spec version the operator was introduced in the operators.Op structure, add to existing new*Op() functions
2. When disassembly happens, opcodes present before the spec version cause an error.
3. Change the ReadModule or DecodeModule function to take a new argument representing the spec version.

I don't think the working group are intending on supporting any non-backwards-compatible proposals, but in the event they do, we can load a different vm. method into vm.funcTable to do the different feature without any performance hit. (that said, this is not likely to ever happen according to the footnote here)

Thoughts?

A good testbed on this might be the sign-extension-ops proposal.