tuple_compare: cleanup key_def virtual func setter

Over time key_def virtual func setter code has turned into a complete
mess: now it's an incomprehensible mix of if-else blocks, linear array
search, and devious array indexing. What is worse, it's duplicated by
tuple_compare_create and tuple_compare_with_key_create. Adding yet
another parameter to key_def templates, which is needed to implement
multi-key indexes, is literally impossible.

This patch attempts to alleviate the situation by splitting code paths
for pre-compiled, plain, and json indexes plus merging tuple_compare and
tuple_compare_with_key setters.