This renames WasmRuntime to ModuleEnvironment, and makes several changes
to allow for more flexible compilation.
ModuleEnvironment no longer derives from FuncEnvironment, and no longer
has the `begin_translation` and `next_translation` functions, so that
independent `FuncEnvironment` instances can operate within the same
module.
Also, this obviates the rest of TranslationResult, as it moves processing
of function bodies into the environment. The DummyEnvironment implementation
gives an example of decoding the function bodies as they are parsed, however
other implementation strategies are now possible.
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