Move the encoding package under rowdata so make it clear how (not) general it is.
Add a replacement encoder factory based on types; make sure that it returns the same answer for all tested types.
merge MMcM: Get rowdata encoding from TInstance

Merge pull request #1121 from MMcM/sql-layer/ais-types3-4
https://github.com/FoundationDB/sql-layer/pull/1121