The new "typed" API that XLA provides for foreign function calls is
header-only and packaging it as part of jaxlib could simplify the open
source workflow for building custom calls.
It's not completely obvious that we need to include this, because jaxlib
isn't strictly required as a _build_ dependency for FFI calls, although
it typically will be required as a _run time_ dependency. Also, it
probably wouldn't be too painful for external projects to use the
headers directly from the openxla/xla repo.
All that being said, I wanted to figure out how to do this, and it has
been requested a few times.
