Improve comment and tests from previous commit, fix lp#1373702.
sort out fixnum / float leaks in split-seconds-for-sleep

Two different (but related) errors in host vs target fixnums
fix a more unusual host compiler leak

a macroexpand-time computation involving the host's value of
internal-time-units-per-second: needs to be done instead with
the target's.
Implement CHAR/=,CHAR-NOT-EQUAL using same logic as numeric /=
Fix disappearing docstring that also broke #!-sb-doc badly. lp#1381781