Feature: "STRING" or "STRINGS"

[JONL at MIT-MC (Jon L White)]

Multics maclisp has used (STATUS FEATURE STRING) to indicate the
presence of the string data type, whereas the LISPMachine uses
(STATUS FEATURE STRINGS).  Rees and others and I propose that NIL
have the status feature "STRING" to indicate the presence of all
the NIL string primitives, as described in MC:NIL;NEWFUN >, and
have the status feature "STRINGS" to indicate the additional presence
of the named functions found on pages 79 thru 83 of the LISPM manual.
The maclisp STRING package will begin doing this too.
N.B.: Exceptions - NSUBSTRING wont be done, due to implementational problems
                   STRING wont be done, due to LISPM's inconsistent definition
Reasoning: status feature BIGNUM rather than BIGNUMS, etc for the
           basic data type capability.