BigUint always use u64 as the internal machine unsigned integer

This change allow a speedup of ~1.5 on shootout-pidigits on a i32
system.  `DoubleBigDigit` is used to abstract the internal
unsigned integer used in computation to simplity future
architecture specialization.

`BigDigit::from_uint` and `BigDigit::to_uint` become
`BigDigit::from_doublebigdigit` and `BigDigit::to_doublebigdigit`.

[breaking-change]
auto merge of #13880 : TeXitoi/rust/biguint-always-use-u64, r=pcwalton

This change allow a speedup of ~1.5 on shootout-pidigits on a i32
system.  `MachineUint` is used to abstract the internal machine
unsigned integer to simplity future architecture specialization.