WL#7840: Allow parsing a single expression

This worklog will implement parsing of a bit_expr into an Item tree (Item*)

Background: In legacy .frm files the full partitioning clause 'PARTITION BY <PARTITION_TYPE> (partition_expression) ...' is stored. When a table is opened the partitioning clause is parsed and updated the partition_info object with its pieces, including Item *partition_info::part_expr and Item *partition_info::subpart_expr.

With the new DD all the parsed information pieces are stored in its broken down form except for the expressions, which are stored in text format (not possible to store Item trees in any other format?). But when retrieving this information and create the Item trees for the expressions there are currently no other way than either compose a full 'PARTITION BY ...' clause (or even worse: CREATE or ALTER TABLE statement).

So to avoid storing/generating a bigger string to parse/retrieve the partition expressions, a way to only parse the actual expression string is needed.