cxxrtl: use CXXRTL_ASSERT for RTL contract violations instead of assert.

cxxrtl: use CXXRTL_ASSERT for RTL contract violations instead of assert.

RTL contract violations and C++ contract violations are different:
the former depend on the netlist and will never violate memory safety
whereas the latter may. When loading a CXXRTL simulation into another
process, RTL contract violations should generally not crash it, while
C++ contract violations should.