Make tuple_extract_key support multikey indexes

Add multikey_idx argument to tuple_extract_key and forward it to
tuple_field_by_part in the method implementation. For unikey indexes
pass -1. We need this to support multikey indexes in Vinyl.

We could of course introduce a separate set of methods for multikey
indexes (something like tuple_extract_key_multikey), but that would
look cumbersome and hardly result in any performance benefits, because
passing -1 to a relatively cold function, such as key extractor, isn't
a big deal. Besides, passing multikey_idx unconditionally is consistent
with tuple_compare.