Add warnings to compiler directive docs

Use specialised default return handling code for "return None" in generators and other functions that return Python objects.

Fix link to Pythran manual in the documentation.

C functions without args should be declared as f(void)

Allow multiple inheritance without __dict__

Closes #2075.

Evaluate multiplication of string literals at compile time if the result is short (<= 256 characters).

Docs Update

Fix typo: to -> two

Disable an optimisation that turned out to be a pessimisation (replacing literal lists with tuples).

Fix typos in docs

Increase number of items for which list.extend([...]) is optimised. Benchmarking shows that append() is still faster for 8 items (on an empty list) than extend().
This limit could be increased for larger lists (which grow by a larger amount), but if we have to settle on one value then 8 is good enough, as we can expect literal item lists to be rather short.

Keep "result_is_used" flag across node replacements in type analysis and optimisations to make sure it's still available at code generation time.

Fixes #2059.