Move supporting javascript to abc interpreter repo

There was quite some duplicate code between this repository's interpret.js (used only to test the WebAssembly interpreter) and iTasks's abc-interpreter.js (used to run the interpreter and for JS interfacing). With this MR (and the corresponding iTasks-SDK!284) all general code is moved to this repository and the JS interfacing code remains in iTasks. Hopefully most changes will now involve only one repository.

This MR also includes a change to the prelinked bytecode format: it now uses variable-length quantities to save roughly 70-75% on the final file size.