sql: rename instr to position & add collation usage

Before this patch we had instr() SQL function.
After the patch it is renamed to position() for a better ANSI
compatibility. Also a few things have been changed: arguments
order, allowed arguments types and usage of collations.

Note: after the patch position() syntax is still different
from ANSI. The desirable syntax is POSITION(substring IN string).
It is not possible to implement right now, because in our SQL
grammar we lack expr types. We have only basic 'expr' type and
it is not possible to write unambiguous rule for POSITION IN.
To solve this we have to refactor grammar and add something
like 'string_expr' (as it is done in other DBs grammars)

Workaround for #3933

@TarantoolBot document
Title: instr() is replaced with position()
Name and order of the arguments has changed for a better ANSI
compatibility:
Before: instr(haystack, needle).
After: position(needle, haystack).

Type checking became more strict: before it was
possible to call the function with INTEGER arguments
or with arguments of different types. Now both arguments
must have the same type and be either text or binary
strings.

Before the patch collations were not taken into
consideration during the search. Now it is fixed, and
both implicit (column) collations and explicit
(using COLLATE expression) are used. Single collation which
would be used in function is determined using ANSI
“Type combination” rules.