Adds options to the bigint constructor function `math.bigint()` to
   control whether/how non-integer inputs are rounded or errors thrown,
   and whether "unsafe" values outside the range where integers can be
   uniquely represented are converted. Changes `math.numeric(x, 'bigint')`
   to call the bigint constructor configured to allow unsafe values but
   throw on non-integers (closest approximation to its previous buggy
   behavior).

   Also restores documentation generation for constructor functions,
   and initiates History sections in function doc pages.

   Resolves josdejong#3366.
   Resolves josdejong#3368.
   Resolves josdejong#3341.