Add trait for linear allocation to enforce use of LinearAllocator at compile-time

[ian-h-chamberlain]

Also convert ndsp::Wave to be generic over any linearly-allocated type so that callers could use Vec etc. if they wanted.

This will also be useful for citro3d-rs, e.g. https://github.com/rust3ds/citro3d-rs/pull/42/files#r1477434491