AST: Add sub-Expression nodes to __repr__ for debuggability
Parser test: _assert_(non)?_equivalent can now take expr lists as args
Parser test: Refactoring

* Split precedence and associativity tests to several test functions
* Use new _assert_equivalence methods that accept lists directly
* Introduce some associativity tests for:
  * Float
  * Bool
  * mod
  * power
* Style cleanup when (unpacking, tuples); remove unneeded parens
* Remove redundant non-equivalence tests that are already tested
  due to non-ambiguity of LALR tables
* Add tests for power VS multiplicative operator precedence