[Pitch] 128 bit Integer Types

[taylorswift]

@wadetregaskis

Presumably Encodable and Decodable would be updated, in Foundation, once [U]Int128 is available to them?

we would likely still run into problems with individual decoder implementations, e.g JSON decoders. if i remember correctly, swift-json has a maximum supported precision of UInt64. i imagine these decoders would have to be updated to handle UInt128.