XLA backend based directly on the assignments interface

[lukstafi]

Implement the XLA backend, compiling directly from Assignments.comp. Relevant links:

• https://openxla.org/xla/operation_semantics
• https://github.com/LaurentMazare/ocaml-xla
• https://github.com/openxla/xla/blob/main/xla/hlo/builder/xla_builder.h
• https://github.com/openxla/xla/blob/main/xla/service/hlo.proto