Document binaryen upgrade process

[spotandjake]

We should properly document the binaryen upgrade process to make it easier and prevent mistakes.

Currently, from what I know the process is as follows:

• Update the libbinaryen version in package.json and binaryen.opam
• Diff the current version to the next version.
  • i.e (123 to 124): WebAssembly/binaryen@version_123...version_124
• Check the diff for
  • src/binaryen-c.h changes api changes
  • src/js/binaryen.js-post.js api changes
  • Changes to PassRegistry::registerPasses in src/passes/pass.cpp

Any api's that have been removed from binaryen-c.h should be removed from our binaryen.ml api, you can search the project and remove their corresponding .c, .js, .ml, .mli and test.ml file. Any trivial api's that are added should be implemented, assuming they exist both in the c and js api's (smoke tests should be implemented as well), and the passes should be updated to match binaryen's correctly.

Any larger api's that are added should ideally have a separate pr submitted alongside the release; however, given the amount of work to implement these api's, an issue can be opened instead.