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ARROW-7179: [C++][Python][R] Consolidate coalesce/fill_null

This removes fill_null in favor of coalesce.

A few points:
- This leaves behind the Python aliases for convenience.
- This is about 25% slower than fill_null for the (Array, Scalar) case for fixed-width types. As far as I can see, this is mostly because the handwritten fill_null only unboxes the scalar once (and does so cheaply), while the generic implementation here unboxes the scalar on every loop (the compiler doesn't manage to hoist it) *and* UnboxScalar is relatively expensive (requiring a virtual call). I tried to make UnboxScalar less expensive but couldn't avoid undefined behavior/incorrect results for the DayTimeIntervalType case.
- This specializes all the two-argument cases for fixed-width types but it might not really be worth it except for the (Array, Scalar) case.

Closes #10987 from lidavidm/arrow-7179

Lead-authored-by: David Li <li.davidm96@gmail.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>