[stdlib] StaticBigInt (#1584)
[stdlib] StaticBigInt (#1584)

* [stdlib] StaticBigInt

* [stdlib] StaticBigInt: add feedback

* Assign myself as review manager and link to the pitch thread

* [stdlib] StaticBigInt: suggestions by John McCall

Co-authored-by: John McCall <d279088d79e357a153f8842902bcfc9d630ff5da@apple.com>

* [stdlib] StaticBigInt: suggestions by John McCall

Co-authored-by: John McCall <d279088d79e357a153f8842902bcfc9d630ff5da@apple.com>

* [stdlib] StaticBigInt: generic subscript, etc.

Co-authored-by: John McCall <d279088d79e357a153f8842902bcfc9d630ff5da@gmail.com>
Co-authored-by: John McCall <d279088d79e357a153f8842902bcfc9d630ff5da@apple.com>
apple/swift-evolution,apple/swift-evolution