Override the TCastBase#preferredTarget() for the MString types to
include the length, character set, and collation of the input string. 

Update a few tests to ensure this works as expected.
Add a second RulesTest for the types resolver to verify correctness.
Add README.txt comment about new test.
merge tjoneslo: Typesresolver cast varchar 255

Merge pull request #1305 from tjoneslo/sql-layer/typesresolver-cast-varchar-255
https://github.com/FoundationDB/sql-layer/pull/1305