Use multiple :EXPORT clauses to provide brief documentation for
exported symbols.

It's not clear to me that it improves the generated code much, but
perhaps I'm simply not skilled in reading CCL disassembly.

Peculiarly, this bug did not show up in my earlier tests.

base32, base64, and ascii85 all need to check for particular characters:
CONVERTER logic in the decoding functions was misinterpreted for this
case.  CONVERTER is really for turning elements of the input sequence
into integers and was inappropriately being used to try to convert the
special characters into integers, which didn't work when decoding octet
vectors.

For those checks, explicitly check the type of sequence we're decoding
from and take the appropriate transformation on the special character.

Make sure we run tests that exercise the :ELEMENT-TYPE keyword argument
to ENCODE.  Also make sure that DECODE can handle inputs that aren't
necessarily strings.

This is more consistent with what Common Lisp already
does (e.g. READ-SEQUENCE), as well as being slightly more efficient.