Relax BufferVec's type constraints

[james7132]

Objective

Since BufferVec was first introduced, bytemuck has added additional traits with fewer restrictions than Pod. Within BufferVec, we only rely on the constraints of bytemuck::cast_slice to a u8 slice, which now only requires T: NoUninit which is a strict superset of Pod types.

Solution

Change out the Pod generic type constraint with NoUninit. Also taking the opportunity to substitute cast_slice with must_cast_slice, which avoids a runtime panic in place of a compile time failure if T cannot be used.

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Changelog

Changed: BufferVec now supports working with types containing NoUninit but not Pod members.
Changed: BufferVec will now fail to compile if used with a type that cannot be safely read from. Most notably, this includes ZSTs, which would previously always panic at runtime.

[msvbg]

Checks out! https://docs.rs/bytemuck/1.15.0/src/bytemuck/no_uninit.rs.html#63

[pcwalton]

LGTM