Move assembly of Column default expressions into expression assembler.
Track the target column for INSERT to VALUES along with type.
Add a new expression to represent default value for that column.
Generate using newly moved method.
Add a YAML test for new cases and error.
merge MMcM: Support DEFAULT in VALUES

Merge pull request #1131 from MMcM/sql-layer/sql-insert-default
https://github.com/FoundationDB/sql-layer/pull/1131