build for WedAssembly

[tomjnixon]

todo

• add C++ tests for C API functions (currently only JS)
• fix use of long for 64 bit types in decorrelation filters (see warning from tests)
• represent exceptions in C API, and catch exceptions when enabled
• add more functionality to C API

more ideas for size improvements:

• improve representation in bs2051_layouts.cpp
  • the current structure results in a manual call to the std::string constructor and the Channel constructor (with lots of parameters on the stack), for each channel in each layout
  • write a POD channel struct which can be stored in static data, and a function that makes a Layout from a char* and a list of channel indexes
• drop pre-computed facets, or at least change the representation of facets to be something like vector<vector<int>>, as the constructor is huge and using set is unnecessary
• turn on wasm bulk memory operations

usage

Build with nix (only tested on linux, but may work on osx):

nix --extra-experimental-features "nix-command flakes" build .#libear_wasm

this puts libear.wasm in result/lib/libear.wasm. Alternatively just use this build:
libear.wasm.zip

There is a JS wrapper here: https://github.com/ebu/libear/blob/wasm_build/src/libear.mjs

There are some tests for that here, which show how to load the library (synchronously) and calculate some gains: https://github.com/ebu/libear/blob/wasm_build/tests/test_libear.mjs

To load from a stream in the browser use something like:

const wasm_buf = fetch('libear.wasm');
let lib = await EARLibrary.from_stream(wasm_buf);
...