Add Support for an Arbitrary Precision Integer Type

[InsertCreativityHere]

I propose adding support for signed integers of arbitrary size!

The vast majority of languages I know of support them either at the language level or often with a standard library type:

• C# has BigInteger.
• Go has big.Int.
• TypeScript has BigInt
• Java has BigInteger (Which is what Kotlin uses too).
• Python supports arbitrary sized integers out of the box.
• Rust used to have BigInt in the standard library but they split it off into a separate crate apparently: BigInt

So there are obvious targets for what we could map this type to in each language.
And the implementations to encode/decode these would be trivial to write. It's just bytes and bit-shifts.

Such a data type has obvious utility, and is general-purpose enough to be useful for a variety of situations.
I think these make it a good candidate for a WellKnownType expressed as custom BigInteger in Slice.