fill_null and coalesce are essentially the same kernel, except the former is binary and doesn't support an array fill value, and the latter is variadic and supports scalar and array fill values.

We should consolidate them into one kernel, picking the faster implementation.

Reporter: Ben Kietzman / @bkietz
Assignee: David Li / @lidavidm

Related issues:

• [C++] C++ array kernels framework and execution buildout (umbrella issue) (is a child of)
• [C++] Add a "coalesce" variadic scalar kernel (duplicates)

PRs and other links:

• GitHub Pull Request #10987

_{Note: This issue was originally created as ARROW-7179. Please see the migration documentation for further details.}