Extend wast2wasm4cpp to also work with Rust sources

[axic]

wast2wasm4cpp is a tool proccessing .cpp files, which looks for a specific comment containing wasm source code and updates the binary code below it, e.g.

    /* wat2wasm
    (func (param i64) (result i64)
      (local i64)
      get_local 0
      set_local 1
      get_local 1
    )
    */
    const auto wasm =
        from_hex("0061736d0100000001060160017e017e030201000a0c010a01017e2000210120010b");

Here the source are the lines below /* wat2wasm and, the tool fills in the binary hex string within the from_hex() call.

Change this tool to emit Rust-compatible output if the filename is .rs:

    /* wat2wasm
    (func (param i64) (result i64)
      (local i64)
      get_local 0
      set_local 1
      get_local 1
    )
    */
    let input = hex::decode("0061736d0100000001060160017e017e030201000a0c010a01017e2000210120010b").unwrap();

Please note it must also run cargo fmt after the replacement, just like it runs clang-format for C/C++ sources.