[R6RS] Arithmetic loose ends

[Michael Sperber]

Two very small issues, mostly for Will at this point:

- Months ago, I added `integer-sqrt', but I seem to remember we also
  agreed to put in `integer-length'.  However, I'm not sure. Right?

- Some people suggested extending `log' to accept an optional base
  argument:

http://srfi.schemers.org/srfi-77/mail-archive/msg00439.html

  This makes sense to me.  Any reason why we shouldn't?

-- 
Cheers =8-} Mike
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