Cleanup AbstractStatementSupport implementation

We currently have multiple StatementSupport implementation bases:

• AbstractStatementSupport
• AbstractQNameStatementSupport
• BaseStatementSupport
• BaseQNameStatementSupport

The latter two provide integrated substatement management, hence should end up being the only entrypoints we are really using.

Remove the former two, along with AbstractDeclaredStatement (from SPI), so that we have complete control of substatement creation. Consider promoting Base* to their SPI Abstract* counterparts if it provides a benefit.