Add Support for `int128` and `uint128` Primitive Types

[InsertCreativityHere]

I think we should add support for these types!

Target Language Support

Rust has always supported i128(doc) and u128(doc) primitives types.

C# also has built-in support for this size of integer, since .dotnet 8 apparently:

• https://learn.microsoft.com/en-us/dotnet/api/system.int128
• https://learn.microsoft.com/en-us/dotnet/api/system.uint128

Python just has int which is arbitrarily sized, so no problem there.

Even Swift added support for these in Swift 6.0 (released in 2024):

• https://developer.apple.com/documentation/swift/int128
• https://developer.apple.com/documentation/swift/uint128

Use Case

We actually use i128 in a few places in the Slice compilers, for integer constants which could be either an u64 or an i64; i128 is the only type that covers both of those ranges. Now that we want the AST to be Slice-encodable, having int128 in Slice would be helpful for this.

[externl]

Swift also have 128 bit ints.

https://developer.apple.com/documentation/swift/int128

[bernardnormier]

We could make int128 and uint128 well-known custom types, not new Primitive types.

Well-known types has several advantages over primitive types:

• less need to add additional types for consistency (a larger varint type, a 128-bit float type)
• no reason to support this custom type as the underlying type of enums