Wrap wasm-opt related functions in struct (#687)

* Add `WasmOptHandler`

Just a basic copy-pasta of what's in the existing optimization code

* Move code from `optimize_wasm` into a handler method

* Pull suggestion string into const

* Remove old Wasm optimization functions

* Remove `dest_optimized` argument from `optimize` method

* Move `check_wasm_opt_version_compatibility` into WasmOptHandler impl

I think it makes sense to have this be associated with the handler
instead of just floating around on its own.

* Move the handler code further up the module, before all the functions

* Decrease indentation of error string

* Move `WasmOptHandler` into its own module

* Move `create_executable` test helper to `utils` module

* Switch from `env_logger` to `tracing`

Related PR: #689

* Indent some code