Clean up kernel creation code in `codegen.driver`. Fix and dry-test SYCL codegen.

!449 (merged) accidentally broke SYCL code generation since SYCL kernels do not receive a launch config, but their kernel object expected one since it was being modelled as a GPU kernel.

This MR fixes this, alongside a few other improvements:

• Target.SYCL is no longer a GPU target
• SyclPlatform no longer inherits from platforms.GenericGpu, but directly from platforms.Platform.
• Emit SYCL kernels using codegen.Kernel instead of codegen.GpuKernel
• Combine the various kernel creation utility functions in codegen.driver into a KernelFactory class for cleaner code
• Move creation of Lambdas from Lambda.from_expression to KernelFactory.create_lambda
• Add dry-dock tests for SYCL code generation that only test if the codegen driver runs successfully

Relates to #126