refactor: cleanup KLR definitions

[govereau]

Remove some unnecessary parts of KLR, and reorganize the source files.

[seanmcl]

Why is expr a statement in a language without side effects?

[govereau]

Very good question. The (admittedly bad) answer is that I am not sure KLR is the final step. Maybe it isn't pure, but there is a transform we can do to make it pure. For example, we see stuff like:

# in nki.isa
 def builtin_custom_op(fn_name, file_name, ucode_version, isa_version, srcs, dsts, **kwargs):

If you call this function, it would look like:

def kernel(...):
  ...
  builtin_custom_op("name", "file", 1, 1, a, b)
  return b

Right, now the call to builtin_custom_op would be one of these expression statements. Not sure what to do here? I think we need to look one-by-one at the ISA operations.