Support POSITION with nondeterministic collations

This patch allows using text position search functions with 
nondeterministic collations.  These functions are

- position, strpos
- replace
- split_part
- string_to_array
- string_to_table

which all use common internal infrastructure.

(This complements the patch "Support LIKE with nondeterministic 
collations" but is independent.)

Some exploratory testing could be useful here.  The present test 
coverage was already quite helpful during development, but there is 
always the possibility that something was overlooked.