Use wasm-merge instead of ocaml-offchain Merge module

[hswick]

prepare.js uses a custom Merge module in ocaml-offchain to link the fileystem WASM module to the given WASM module.

Was wondering if we could use the wasm-merge tool that binaryen provides instead of the custom script. From what I've been reading as along as the WASM module explicity imports the filesystem than it should work.

[mrsmkl]

The Merge module might do some special stuff, but worth taking a look.