Various postgresql fixes

Mainly postgresql fixes, but one new Xplain feature: you can now use if statements at some places in a stored procedure.

There are limits, you can't use extend in an if statement as that can lead to all kinds of odd output errors.

GraphQL support

Added several improvements that makes it possible to write Xplain and generate GraphQL schemas and resolvers from it. Most changes are in the xplain2sql.xml file. It has more details, so XSLT based transformations have it easier.