DNA string matching now uses IUPAC groups, i.e. "TTATG" will match
"TNWTG".
Comparison method selection and match highlighting.

As long as IUPAC-aware comparison of DNA-STRINGs is a rather heavy
process, an argument was added to the function DNA-STRING-EQUAL to
select how the DNA strings are to be compared (:WEAK - just plain
string comparison, :STRONG - IUPAC-aware comparison).

PRINT-MATCHES function was changed to use the newly written function
HIGHLIGHTED, which highlights some part of a string during it's
display on the terminal screen.
Some more ABIF improvements:
1. conditions are used to signal invalid file format;
2. docstrings were added where missing, corrected where they were
wrong;
3. entries containing their own values are handled correctly;
4. before reading contents of entries found in the directory, we sort
them by their offset in the input file.
More exported symbols from the GAS-ABIF package.