tracking: 2.0

[DaniPopes]

I think 2.0 is pretty far away since we're blocked on generic const expressions (GCE), but I think it's worth gathering and discussing all the ideas in one place.

• Remove Bits type
  • This is a thin wrapper around Uint which does not have any extra functionality
• Remove second Uint generic (LIMBS), blocked by GCE
  • It could also be a non-breaking change by using a default value
• Remove support for BITS == 0 and possibly BITS == 1
  • I don't see the point of U0, maybe U1 too. It only causes unnecessary cruft throughout the codebase, where we check for BITS == 0 and have a special case path because we have 0 limbs
• Represent Uint internally using machine word-sized Word/DoubleWord type aliases instead of hardcoded u64
  • Support for 32-bit word width #371
  • possibly also configurable with compile time features, not just target word size
• Align parsing behavior with standard library
  • Empty string gets parsed as 0 #348
• Make as_le_slice and similar methods return an array of [u8; Self::BYTES]
• Remove support for old crates support features
  • ssz, ark-ff-0.3 ...
• Resolve the overflow vs panicking arithmetic question: Make UInt panic on overflow if overflow_checks is enabled #408, Add feature to enable panicking arithmetic #522

cc @mattsse @gakonst @recmo @prestwich

[prestwich]

I'd like to consider

• an interface review. do our function names and behaviors conform to those of the std library and other integer libraries
• variable length uints