Added WebIDL bindings for `GPUCommandEncoder`, `GPUCommandBuffer`, `GPUComputePassEncoder`, `GPUProgrammablePassEncoder`.
Implemented the `beginComputePass`, `copyBufferToBuffer` and `finish` functions of `GPUCommandEncoder`.
Implemented the `createCommandEncoder` function of `GPUDevice`.
Added WebIDL bindings for `GPUBindGroupLayout`, `GPUBindGroupLayoutDescriptor`, `GPUBindingType`, `GPUShaderStage`
and `GPUBindGroupLayoutBinding` (Note: The servo's codegen doesn't like the name, because its already occupied).
Implemented the `createBindGroupLayout` function of `GPUDevice`.
Added WebIDL bindings for GPUBuffer, GPUBufferDescriptor, GPUBufferUsage
Implemented the `createBuffer` and `createBufferMapped` functions of GPUDevice
Added the WebIDL bindigs for GPUDevice, GPUObjectDescriptorBase, GPUDeviceDescriptor, GPUObjectBase
Implemented the `requestDevice` function of `GPUAdapter`