Memory barrier documentation, and wrappers for common usage

[hrydgard]

In https://embarkstudios.github.io/rust-gpu/api/spirv_std/arch/fn.control_barrier.html , there's no explanation of the MEMORY template parameter, or even what values are valid in it. (Turns out it is a scope, just like the EXECUTION parameter).

I guess what would be most immediately helpful is basically just examples matching the common ones like HLSL's GroupMemoryBarrierWithGroupSync.

[hrydgard]

Here's an exact implementation of GroupMemoryBarrierWithGroupSync(), verified against the output of DXC:

    unsafe {
        control_barrier::<
            { Scope::Workgroup as u32 },
            { Scope::Workgroup as u32 },
            { Semantics::WORKGROUP_MEMORY.bits() | Semantics::ACQUIRE_RELEASE.bits() },
        >();
    }

Maybe we should have a library of wrappers for common combinations.