Forbid unsized fields in patterns of binary generators

It makes no sense to be able to do `<<...,Rest/binary>> <= ...` in a
comprehension.
Simplify v3_core's translation of bit string generators

Now that unsized binary segments are forbidden in patterns of bit string
generators, v3_core:append_tail_segment/2 does not need to check for an
existing unsized tail segment and can thus be changed to a simple '++'/2
call.