bitvec: Test and fix regression for C++->C conversion

bitvec_read_field/bitvec_write_field in the PCU used a C++ reference
and when porting to C it was decided to pass the parameter by value
and this lost the "back propagation" of the new index. Change the
parameter to be an in/out parameter and this way do not have a silent
semantic break in the osmo-pcu (where we copy the reference in csn.1
by value) and have a true compile failure.

Add Max's simple test for bitvec_unhex function leaving the checking
of bitvec_read_field and the side effect in the datastructure about
the number of bits still open.
4 files changed