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ARROW-14244: [C++] Reduce scalar_temporal.cc compilation time

* Split unary and binary kernels into two distinct translation units
* Fix temporal codegen infrastructure so as not to generate code for kernels
  for unused argument types (e.g. avoid generating code for time inputs
  if a kernel is only defined for date types)

Closes #11353 from pitrou/ARROW-14244-scalar-temporal-codegen

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>