Use naga::Module as a ShaderSource for non web targets

[Gordon-F]

Is your feature request related to a problem? Please describe.
Use naga::Module as a ShaderSource for non web targets like spv.

Describe the solution you'd like
Accept naga::Module in ShaderSource. wgpu-core already accept it.

wgpu/wgpu/src/lib.rs

Lines 732 to 745 in 0c76819

	pub enum ShaderSource<'a> {
	/// SPIR-V module represented as a slice of words.
	///
	/// wgpu will attempt to parse and validate it, but the original binary
	/// is passed to `gfx-rs` and `spirv_cross` for translation.
	SpirV(Cow<'a, [u32]>),
	/// WGSL module as a string slice.
	///
	/// wgpu-rs will parse it and use for validation. It will attempt
	/// to build a SPIR-V module internally and panic otherwise.
	///
	/// Note: WGSL is not yet supported on the Web.
	Wgsl(Cow<'a, str>),
	}

Describe alternatives you've considered
Use naga to produce wgsl or spv from IR in user projects.

[kvark]

I think that would be fine from the API point of view. Now that there is a WGSL backend, we can generate the shader for the web target internally.
However, it may be less usable than it seems: naga modules are not clone-able. So if @Frizi wants to try it out and report back, we can use this input to figure out if this can be reasonably exposed.

[daxpedda]

This can be closed I believe.
Has been implemented by #2801. I also made Module cloneable in gfx-rs/naga#2013.