Tracking issue for num-bigint 0.3

[cuviper]

• Update MSRV to 1.31, needed to deal with rand's use of package renaming
  • Update to 2018 edition and clippy fixes #130: Convert to 2018 edition too
• Finish i128 support #62: Finish support for 64-bit BigDigit
• Use the alloc crate for no_std (Rust 1.36+) #101: Enable no_std with the alloc crate (MSRV 1.36)
• Add PartialEq and PartialOrd between built in integer types and BigInt/BigUint #105: Add comparisons with primitive integers (affecting type inference)
  • Reverted in Revert PartialEq and PartialOrd with primitives #151, as it broke a lot of integer type inference.
• implement Pow for BigInt #106: Implement Pow by value (affecting calls that currently use references)
• Bump dependencies #110: Update to rand 0.7 (MSRV 1.32) and quickcheck 0.9 (MSRV 1.34)
• Reconsider {Div,Rem}::Output with primitives to match the smaller type (related to Add new traits for a faster and type safe Div/Rem #88)
  • Postponing.

Other PRs and issues may be included too, but these are the ones that impact compatibility.