pidigits resurected

Changes:
* default value when no args
* license
* removed libc printing
* use extra::bigint instead of handmade gmp binding
auto merge of #10555 : TeXitoi/rust/pidigits-resurected, r=alexcrichton

Changes:
* default value when no args
* license
* removed libc printing
* use extra::bigint instead of handmade gmp binding

The drawback is that it's 2 order of magnitude slower, the good news is that there is a good bench for extra::bigint.