[iterator.traits] An implementation that provides __far pointers doesn't need
the standard to tell it (non-normatively) how to implement them.

Fixes #260.
[utilities] Use `{}` rather than ` = T()` to default-construct
piecewise_construct and allocator_arg.

Fixes #250.
[basic] Fix inconsistency between here and [temp.type]p1: define when two
template-ids referring to variable template specializations are the same.
This is the clear and unambiguous intent.

Fixes #239.