Add WASM/WASI support

[repi]

This adds support for building the lib for both wasm32-unknown-unknown and wasm32-wasi with single-threaded execution, and for running the CLI with wasm32-wasi.

Example:

rustup target add wasm32-wasi

cargo build --release --target wasm32-wasi

wasmer run ./target/wasm32-wasi/release/texture-synthesis.wasm --mapdir=out:out --mapdir=imgs:imgs -- --out out/01.jpg generate imgs/1.jpg

It is not too practical yet without multithreading support (which WASM doesn't support yet), but is very portable and is a good benchmark to test WASM performance with.