base: Fix assigning between identical bitfields.

base: Fix assigning between identical bitfields.

If two bitfields are of the same type, also implying that they have the same
first and last bit positions, the existing implementation would copy the
entire bitfield. That includes the __data member which is shared among all the
bitfields, effectively overwritting the entire bitunion.

This change also adjusts the write only signed bitfield assignment operator to
be like the unsigned version, using "using" instead of implementing it again
and calling down to the underlying implementation.