Float % and // wrong with huge operands; #150 follow-up nits #151

From Motoko's #150 review. This happens on base too: (% MaxInt64 1e33) gives 1.0000000000000092e+33, and // gives -1.

Nits:

• The error text for non-numeric operands to % and // is now '< compares incompatible types'.
• The destructure VALUE_ERROR has no line or column.
• Document the instruction cost: about 2.3x for small % and //, 13.4k for to_base at MinInt64 in base 2.
• The construct-table notes still say 'via to_int(/)'.