This patch adds support for functional indexes by creating an indexed hidden
generated virtual column for each functional key part specified by the user. A
functional index can be specified by using an extra set of parenteses around
each key part: CREATE INDEX func_index ON t ((ABS(col1)), col2, (col3 + col4));
By re-using the functionality for indexed generated columns, the server already
knows how to use any functional index if applicable.

Generally, functional indexes follows the same rules and limitations as indexed
generated columns (indexed expressions must be deterministic etc.). An extra
column is added to EXPLAIN output,INFORMATION_SCHEMA.STATISTICS and SHOW KEYS
output that prints the expression of the relevant index.

Change-Id: Ib74077dc711dbaf5cf64b5918b21d269a0459499