Make TryFromBytes a super-trait of FromZeros

[jswrenn]

The TryFromBytes derive for unions requires that Self: NoCell, due to the safety obligations of Ptr::project in its implementation of is_bit_valid. By contrast, the derives for FromZeros and FromBytes on unions do not require Self: NoCell.

We would like to make TryFromBytes a super-trait of FromZeros, but (as we discovered in #952) this mismatch in requirements presents us with a trilemma:

• don't make TryFromBytes a super-trait of FromZeros
• find a way to remove the NoCell requirements on the TryFromBytes derive for unions
• propagate the NoCell requirement on unions to the derives for FromZeros and FromBytes

Short-Term Resolution

We:

• don't formally make TryFromBytes a super-trait of FromZeros, but we document that this relationship may be required in the future
• propagate the NoCell requirement on unions to the derives for FromZeros and FromBytes

Making FromZeros: TryFromBytes may create future breakage for customers manually implementing FromZeros, but this workflow is explicitly documented as unsupported.

[joshlf]

Closed by #952