This is a regression after MDEV-13671.
The bug is related to key part prefix lengths wich are stored in SYS_FIELDS.
Storage format is not obvious and was handled incorrectly which led to data
dictionary corruption.
SYS_FIELDS.POS actually contains prefix length too in case if any key part
has prefix length.
innobase_rename_column_try(): fixed prefixes handling
Tests for prefixed indexes added too.
Closes#1063
802ce9672f