Provide some way for C & JS to visit and transform module code

Currently, Binaryen uses custom passes to transform Wasm (e.g. in wasm-merge). These passes are unavailable to C and JS, meaning that there's no built-in way for the users to transform the code.
In my project, I need to transform the code using Binaryen.js. Currently, I recursively gather all the information about expressions and reconstruct them with the required changes. My code is giant and repetative, as there's no way to convert from the JS object info format back to the original (or modified) expression.
I have two suggestions: either provide a way for JS to reconstruct expression from the corresponding info object, or provide a way for C (and add the corresponding JS bindings) to transform the code using callbacks or some other API.
I am willing to open a PR if we discuss this idea further!

Start by reading src/tools/wasm-merge.cpp and the existing Binaryen.js bindings to understand how custom passes and expression information are exposed. Compare the two proposed directions—reconstructing expressions from JS objects or adding C callbacks with JS bindings—and clarify the API scope before implementation. Done means C and JS users can visit and transform module code without manually rebuilding expressions.