Rename `LayoutVerified` to `Ref`

[joshlf]

The LayoutVerified type wraps a byte slice reference. It is a witness type that guarantees that the length and alignment have been validated to be compatible with a type parameter, T. It's named based on this witness role (the layout has been "verified"), but the name is confusing for consumers. We should rename it to Ref or similar - it's a reference type that enables zero-copy operations, hence zerocopy::Ref.

[MSalah73]

I'll take this one

[joshlf]

Hi @MSalah73 , this one isn't ready for contribution yet unfortunately - I'm going to need to go through and make sure that this change doesn't break anything in Fuchsia. Once I've done that, I'll add the good first issue Good for newcomers label. In the meantime, check out that label for stuff that we need help with!