Added a specialized converter for ldexp
Variable renaming in function converter
Extract method raise_type_error

Extracted the else branches of converter and ldexp_converter into a function
named "raise_type_error"
Removed comment code
Added new test test_argument_error

It checks if the ArgumentError wins over the TypeError if too few arguments,
which also have the wrong type, are passed to ldexp, hypot and atan2.
Corrected type signature of method_ldexp
Fixed the ldexp converter...

...but the fix is kind of a hack.