workspace: Remove wasmparser::Operator type usage

[Robbepop]

With #697 merged it is possible to remove the Operator type from the wasmparser crate.

The new VisitOperator trait allows to parse and validate Wasm without the need for matching on an Operator. Instead respective visit methods are invoked which avoid unnecessary unpredictable branches and is therefore the preferred and recommended API to use for high-performance Wasm parsing and validation.

This issue tracks progress and discussion for removing the Operator type.

cc @alexcrichton

ToDo

• Remove Operator type in the wasmparser crate.
  • ConstExpr validation: Remove the usage of Operator in const validation #737
  • enum Operator
• Remove Operator type in the wasmparser-dump crate.
  • Remove usage of Operator in wasm-tools dump #712
• Remove Operator type in the wasm-mutate crate.
• Remove Operator type in the wasmprinter crate.
  • Migrate wasmprinter to visit_with_offset #721
• Rewrite existing examples in terms of the new VisitOperator API.
• Adjust docs for usage information about the new VisitOperator API.

[alexcrichton]

I'm going to actually go ahead and close this despite not being fully finished yet. With the for_each_operator! macro it's trivial to maintain both an enum-based representation in addition to the trait-based visitor. Some use cases I think are still better expressed with an Operator enum than the trait (so long as performance isn't a concern), so it seems reasonable to me to have both in this library.