Now uses specialised functions for complex abs

[da-woods]

This patch avoids abs(a_complex_number) casting the complex number back to a Python object and instead uses the __pyx_c_abs* functions that are already defined. It also modifies the specialization for abs on int and long to not require the GIL.

Also added a test case to confirm it's working

Notes:

• I don't think the correct utility code is loaded for complex abs
  (but it should be loaded by the complex data types hopefully)
  This might be worth someone double checking
• Would probably be worth making double and float abs not require GIL
  (but this is a minor improvement to existing code)

[scoder]

Seems worth putting this in a list comprehension by now.
And yes, the utility code will get loaded automatically by the type declarations in user code.

[da-woods]

I've updated it to use generator expressions to reduce the code duplication. (I've used generator expressions with list() instead of a list comprehension to avoiding adding the iteration variables to the module).

I've done it for both my new code (for complex numbers) and the existing code (for integer types). The integer types are a bit more messy since they needed strings and getattr on the PyrexTypes module to work, so it's possible you may not want to keep this.