Deduplicate translation of constant expressions and general operators in func_environ.rs

In https://github.com/bytecodealliance/wasmtime/pull/13487 the processing of `ConstExpr` has moved from the runtime to compiled code. As pointed out [here](https://github.com/bytecodealliance/wasmtime/pull/13487#discussion_r3313862045), however, this results in some duplication of processing `ConstOp` internally within Wasmtime. It should in theory be possible to more-or-less implement `From for wasmparser::Operator` and reuse the existing operator translation in `FuncEnvironment` instead of duplicating it in `translate_const_expr`. This isn't being done in #13487, but it's still a worthwhile thing to look into in the future. The main blocker right now is creation of the `FuncValidator` which `translate_operator` requires, likely resulting in modifications to `wasmparser` to create a special kind of validator for const expressions (or something like that)