Add atomic.Uint8 and atomic.Uint16 support

[AdamMagued]

Summary

Adds atomic.Uint8 and atomic.Uint16 types to go.uber.org/atomic, providing type-safe atomic operations on 8-bit and 16-bit unsigned integers.

Because standard library sync/atomic only provides primitives for 32-bit and 64-bit words, Uint8 and Uint16 are backed by Uint32 with compare-and-swap (CAS) loops for arithmetic modifications, ensuring accurate unsigned overflow and wrap-around semantics.

Changes

• Add Uint8 and Uint16 atomic wrappers with generation directives using gen-atomicwrapper.
• Provide full API parity matching existing integer atomics:
  • NewUint8, NewUint16
  • Load, Store, Swap
  • CompareAndSwap, CAS
  • Add, Sub, Inc, Dec (via lock-free CAS loop)
  • String
  • MarshalJSON, UnmarshalJSON
  • MarshalText, UnmarshalText
• Update non-comparability tests in nocmp_test.go to cover Uint8 and Uint16.
• Add concurrent stress test cases in stress_test.go.
• Add comprehensive unit tests in uint8_test.go and uint16_test.go verifying 100% statement coverage.

Fixes #123

[CLAassistant]

All committers have signed the CLA.