Created star expression parser test. For both isolated expressions and within the index of for-statements.
Added parsing of star expressions
Added semanal tests for star expressions
Added semantic analysis of starred expression
Rudimentary type checking of star expr: Added check for nr lvalues vs rvalues
Type checking of assignment to starred expressions
Whitespace fixes
Added semanal tests for starred expressions in for-stmt and generator expressions
Changed index of for-stmt and generator expressions to be a single node (possibly a tuple) instead of a list. Updated tests accordingly.
Added type inference tests for starred expressions in assignment targets
Refactorred duplicate code
Removed explicite check for parenthesis from parsing of index variables (e.g. of for-stmts)
Added StarType, parsing of it, and check that only a single star type is present in a tuple
Parse and semanal tests for star type annotations. Fixes missing line number in type annotation error message.
Added semanal check for starred type annotations
Added type check test for annotated starred expressions.
Added documentation for starred types
Fix overload resolution with ducktype compatibility

Ignore ducktype compatibility (e.g. int being compatible with float)
when determining which overload variant to pick, since ducktype
declarations are not honored by isinstance().

Fixes #410.
Update docstring
Fix overload resolution