Normalize grammars - 2 (#1420)

Normalize grammars - 2 (#1420)

This is another step towards addressing #1304 and #1344. This pull request:

- Refactors SygusGrammarNorm
- Makes SyGusGrammarNorm a default component in the construction of grammar. The linearization of grammars however only occurs if the option --sygus-grammar-norm is used.
- Performs a "chain transformation" in the application of the PLUS operator in integer grammars
- Removes redundant expansions of definitions from TermDbSygus
- Adds a default empty print callback to SygusEmptyPrintCallback

This lays the basis for more general linearizations.