systemc: Fill out some of the sc_vector.hh classes.

systemc: Fill out some of the sc_vector.hh classes.

The iterator class needs to have some operators, etc., defined so that
tests written against it can compile correctly. The implementations
were heavily influenced by the Accellera implementation.

Also it should be noted that the systemc spec is incorrect where it
defines these classes. When implemented like they are in the
Accellera version, the versions of bind in sc_vector_assembly which
take sc_vector<T>::iterator and iterator parameters are different, and
so they can overload each other. If implemented as described by the
spec however, those types are the same and the code won't compile.

Change-Id: I62852adf7e6c0a2f3df076ba4e93d2501859c32d
Reviewed-on: https://gem5-review.googlesource.com/11177
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>