optimise away redundant None and type checks

[scoder]

After implementing #233 and fixing #166, the syntax tree ends up with a couple of redundant None checks where method calls were optimised. The C compiler might be able to remove them in many cases, but there should still be a separate pass that removes the redundant checks right before the code generation phase.

[scoder]

Cython 0.13 has a new generic method ExprNode.as_none_safe_node() that injects a NoneCheckNode only when it is required (e.g. not for literals). This can be extended to include value ranges found by a future control flow analysis step. For now, it avoids truly redundant checks effectively.