Make int-to-string conversion a primitive in CakeML

This issue is about adding semantics to the source primitive `FromTo IntT StrT` such that it converts any integer into the corresponding string, e.g. 12 becomes "12" and -5 becomes "-5". (Don't generate "~5" for negative 5.)

The compiler should implement this efficiently also for very large integers. Ideally, this int-to-string conversion should be directly supported by CakeML's verified bignum library.

Start by tracing the source primitive FromTo IntT StrT through the compiler and locating the verified bignum library. Done means integer values convert to decimal strings, negative values use a minus sign rather than "~", and very large integers are handled efficiently with verified-library support.

Written by the indexing model from the issue text.