This PR upstreams some UInt theorems from Batteries and adds more
`toNat`-related theorems. It also adds the missing `UInt8` and `UInt16`
to/from `USize` conversions so that the the interface is uniform across
the UInt types.
**Summary of all changes:**
* Upstreamed and added `toNat` constructors lemmas: `toNat_mk`,
`ofNat_toNat`, `toNat_ofNat`, `toNat_ofNatCore`, and
`USize.toNat_ofNat32`
* Upstreamed and added `toNat` canonicalization; `val_val_eq_toNat` and
`toNat_toBitVec_eq_toNat`
* Added injectivity iffs: `toBitVec_inj`, `toNat_inj`, and `val_inj`
* Added inequality iffs: `le_iff_toNat_le` and `lt_iff_toNat_lt`
* Upstreamed antisymmetry lemmas: `le_antisymm` and `le_antisymm_iff`
* Upstreamed missing `toNat` lemmas on arithmetic operations:
`toNat_add`, `toNat_sub`, `toNat_mul`
* Upstreamed and added missing conversion lemmas: `toNat_toUInt*` and
`toNat_USize`
* Added missing `USize` conversions: `USize.toUInt8`, `UInt8.toUSize`,
`USize.toUInt16`, `UInt16.toUSize`
4969ec9cdb