[fx] Use module Operations instead of Module.

[stellaraccident]

This was only used in certain advanced uses of the API that want to build into their own module. The MLIR Module class is an awkward/restrictive way to require this to go as only some things will have it. Just switch everything to be based on a module Operation.

Also adds an optional visibility specifier to entry points that construct a function.

[dan-garvey]

Looks like there is some underlying execution engine change needed to support operation versus module?

[stellaraccident]

Ugh. There is stilly tech debt in the ExecutionEngine that needs to be excised...