Audio file sampler implementation

[tomoyanonymous]

This PR adds audio file sample using symphonia crate.

This PR also introduces ExecContext, a pair of the compiler and VM, to manage type information of external functions.
ExecContext is needed for decoupling external functions which depend on a external crates.

The PR contains an implementation of array access parser but I'm going to change the way of loading & reading wav files into that uses higher-order external function.

//loadwav returns higher-order function that takes playback position in samples as an argument
let sampler = gen_sampler_mono("test.wav")
fn counter(){
    self+1
}
fn dsp(){
    counter() |> sampler
}

• only mono files are supported(other format causes panic on runtime)
• index for sampler is a raw position of samples(Now, samplerate mismatch between audio driver and audio file are not handled. Resampler is not implemented as a part of symphonia, but there's an example code of resampler in symphonia-play application, it can be derived.
• index are not needed to be integer, a boundary check and linear interpolation are performed on read.(Aceess out of range returns 0.0 .)
• gen_sampler_mono function internally creates external Rust closure that holds Vec as internal data(The allocated external closure will never be released in the current implementation. )

Todo:

• add api for wrapping an external closure with mimium closure
• String literal type support
• Relative path resolution from each mmm files
• add test for file reader

[tomoyanonymous]

I think it's finally ready to merge.

A new api in the VM, wrap_extern_closure is introduced to call higher-order native closure.

    let res = move |machine: &mut Machine| -> ReturnCode {
        let pos = vm::Machine::get_as::<f64>(machine.get_stack(0));
        // this sampler read with boundary checks.
        let val = interpolate_vec(&vec, pos);
        machine.set_stack(0, Machine::to_value(val));
        1
    };
    let ty = function!(vec![numeric!()], numeric!());
    let idx = machine.wrap_extern_cls(("loadwavfile_impl", Arc::new(res), ty));
    machine.set_stack(0, Machine::to_value(idx));
    1
}

wrap_extern_closure internally modifies VM program on the fly (generate a glue code which calls Rust closure), so now program is now a member of VM struct. This change made initialization step of VM complicated, I will refactor it later in some way.

Other next todos:

• Move common test modules into an independent crate so that each external crates can use it and test can be defined within each modules(now the test for mimium-symphonia uses duplicated codes.)
• Relation between ExecContext and Driver & RuntimeData in mimium-audiodriver are total mess, should be restructured