Implement GPU types and methods

[dfellis]

The exact shape of this isn't super clear, yet, but either a set of opaque types and bound functions, or pure Alan code on top of a thin wrapper over wgpu/WebGPU is needed to construct an AST to serialize to wgsl and then execute on the GPU.

This is the purpose of Alan v0.2

[dfellis]

Remaining built-in functions of wgsl to bind using this as the reference:

• countLeadingZeros
• countOneBits
• countTrailingZeros
• cross
• degrees
• determinant
• distance and length (these will definitely get different names, though)
• dot
• exp2 (maybe? Not sure if I need to include this when pow exists. I did include exp but only because that's actually pretty commonly used)
• extractBits
• faceForward (this appears to be a composite function on its own, unsure if I should include it)
• firstLeadingBit
• firstTrailingBit
• fma
• fract
• frexp (including it in the list as considered, but I don't see any value in it)
• insertBits
• inverseSqrt
• ldexp (also doesn't seem necessary? it's the inverse of frexp but I don't see the expressive value here)
• mix
• modf (also weird to include when the other functions already exist)
• normalize
• radians
• reflect
• refract
• reverseBits
• round
• saturate
• sign (do we need this?)
• smoothstep (also a compound function, but if its backed by hardware maybe?)
• step
• transpose
• pack4x8snorm
• pack4x8unorm
• pack2x16snorm
• pack2x16unorm
• pack2x16float
• unpack4x8snorm
• unpack4x8unorm
• unpack2x16snorm
• unpack2x16unorm
• unpack2x16float
• storageBarrier (useful for loops on mutable storage where different "phases" of the shader need to see the results of the prior "phase" from more than one thread)
• workgroupBarrier (not including this at first, as I have no way to represent the workgroup pointer at the moment)
• workgroupUniformLoad (not including this at first, as I have no way to represent the workgroup pointer at the moment)

I am not including any of the derivative functions because they're only for pixel shaders, I'm not including any of the texture functions because I do not have any texture functionality at the moment (and I hope to get away with just the copy-buffer-to-shader functionality to do rendering), and I'm not including any atomic operations at the moment as I have no representation of atomics.

This is just the list of things I want to get done for compute shaders, which are the only shaders I'm exposing at the moment.

I also still feel like there's some work to be done on the GBuffer type and its usage, but I'm not going to consider that blocking to close this issue out.