Create new bindings for the referenced register upon entry to subpattern-reference.

When entering a register x via subpattern-reference, the registers
local to x receive new dynamic bindings, which shadow the old bindings
for the duration of the subpattern call.  Previously, "local" did not
include the register itself--x in this case.  With this patch, the
referenced register now receives a new binding as well.

It's not entirely clear that this is the appropriate behavior.  In a
regex like "(.\1?)(?1)", the back-reference to '\1' now will always
fail, rather than potentially matching according to what was matched
in the first pass through the first register.