Small features in BipFunction, BipElt, BipStruct and BipType

This PR includes the following changes:

For BipStruct, BStructMember and BipType:
- add a size setter for BStructMember
- add an exists class method for BipStruct
- add class methods get_by_regex and get_by_prefix for BipStruct
- add faster way to get the BipStruct from a BTypeStruct (struct and final_name properties)
Add goto method for BipFunction
Add get_by_regex, iter_by_regex, iter_by_prefix and get_by_prefix for BipElt
Add tests for all of those
Add python3 compatibility layer to hx_cfunc.py
Fix a test for python3 which where not working correctly

synacktiv / bip