Exact remainders; refuse numbers held rounded (closes #3)

x % y is now y times the fractional part of x/y. The old form, x minus
y times the whole part of x/y, passed through a product that go/constant
could hold only rounded even when both operands and the remainder were
exact, and then replied 0.

A number whose numerator or denominator reaches 4096 bits, which
go/constant holds rounded, is now refused as too large wherever it
appears, literals included. A sum of such numbers could lose the answer,
and a rounded exponent near a whole number was computed as an exact
power. This replaces the separate check on a negative base's exponent.

Model: opus-5-5

README.md

@@ -176,7 +176,10 @@ container.
   Whole numbers below 10<sup>21</sup> are written exactly; other results
   in the shortest form that reads back as the same double, in exponent
   notation from 10<sup>21</sup> up and below 10<sup>-6</sup>. A result
-  beyond the range of a double is refused as too large.
+  beyond the range of a double is refused as too large, and so is any
+  number, even a small one such as `1e-1300` or one inside a longer
+  expression, whose numerator or denominator reaches 4096 bits:
+  `go/constant` could hold it only rounded.
 - **Failure is an exit.** If the chat client exits or the connection to
   it drops, the bot exits with an error and the container's restart
   policy starts both again. `SIGTERM` stops the bot, which stops the