Remove obsolete symbol names from package-data-list
Change WRITE-TO-STRING from FLUSHABLE to UNSAFELY-FLUSHABLE.

An "unused" call of the function might have been intended to signal a
condition about unprintable objects, or perform random side-effects
in user-defined printers. But in practice, calls were not flushed,
because if the compiler-macro worked, the transform to STRINGIFY-OBJECT
was not flushable.

Incidentally, make STRINGIFY-OBJECT a known fun so that the compiled code
for the expansion of WRITE-TO-STRING avoids using RETURN-MULTIPLE.
Remove cruft from GENSYM and make it significantly less consy usually.

There was at one point a QUICK-INTEGER-TO-STRING function
which Nikodemus removed due to it being somewhat unsightly -
it computed an approximate string length and then used %SHRINK-VECTOR.
This change computes a correct length, and avoids UCS4 when possible.