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ARROW-13365: [R] bindings for floor/ceiling/truncate

This also arranges the function mappings in `.unary_function_map` in alphabetical order within categories and adds some comments in `expression.R`

Closes #10745 from ianmcook/ARROW-13365

Authored-by: Ian Cook <ianmcook@gmail.com>
Signed-off-by: Ian Cook <ianmcook@gmail.com>