Use runtime's string ports.

Use runtime's string ports.

Also read file in one swell foop, rather than character by character
in a slow painful chain of indirections thousands of times slower.

We'll use legacy strings here because this code was all written under
the assumption that `string' meant byte vector, and legacy strings
still have that semantics.

Eventually this should all be adapted to use bytevectors instead for
storage, and interpret things as strings only as necessary for text
processing beyond mail data formats.