Generalize the core from_hex() to take the input as pair in input
iterators. The optional 0x prefix is still omitted but the "skip space"
feature has been dropped.

This adds the filter_iterator input iterator adapter to filter input
values. With the additional is_not_space predicate these restores
the option to filter-out whitespace before parsing hex.
This is useful e.g. when loading hex from a file.