Refactor `ED_object_parent_set`:
- Mark parameters `ob` and `par` as `const` so that it's clear the
function doesn't assign any other value to them.
- Rename `pararm` to `is_armature_parent`; I mis-read it as `param` all
the time, and it was very confusing.
- Replace repeated `if-else` statements with `switch` statements.
- Reorder preconditions to have some simple checks first.
- Flip condition on a huge `if`-statement to return early and unindent
the remainder of the function.
This function still requires splitting up into smaller functions, but at
least this is a step forward.
No functional changes.
b8d4a2aff8