Add Specifier trait implementation for nonzero primitives

[ollie-etl]

Adds Specifier trait implementations for non zero primitives, an Option types containing non-zero primitives This allows Option<NonZero<u16>> etc to be used in bitfield specification directly.

[ollie-etl]

This approach was chosen over the blanket implmentation for NonZero types

impl<T: Specifier + ZeroablePrimitive> Specifier for NonZero<T> {
  // ...
}

That approach requires the unstable language feature ZeroablePrimitive to be enabled, whereas the provided implementation works on stable

[csnover]

Hi! I am going through and triaging old PRs. Are you still interested in working on this and would like a review? Thanks!