This adds a simple build script to automatically build all Rust sourced Wasm benchmarks.
Also this moves the generated Wasm files into their respective source directories as out.wasm.

This big advantage is that this heavily simplifies building the Wasm benchmarks and avoids having developers to install tools such as wasm-opt manually on their machines.
Also this tool automatically detects all Rust sourced Wasm benchmarks and automatically builds when when invoked.

Example run:

> cargo run --package build_benches             
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.09s
     Running `target/debug/build_benches`
tiny_keccak:
    building .. done
    optimizing .. done
    finalizing .. done
reverse_complement:
    building .. done
    optimizing .. done
    finalizing .. done
regex_redux:
    building .. done
    optimizing .. done
    finalizing .. done