Multi-precision Integer extension for mruby

This extension uses fgmp, which is a public domain implementation of a subset of the GNU gmp library by Mark Henderson markh@wimsey.bc.ca. But it's heavily modified to fit with mruby. You can get the original source code from https://github.com/deischi/fgmp.git. You can read the original README for fgmp in README-fgmp.md.

If you want to create your own Multi-precision Integer GEM, see examples/mrbgems/mruby-YOUR-bigint/TODO-HINT.md.