Generalise SIMD casting to unequal bitwidths.
Generalise SIMD casting to unequal bitwidths.

graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,dwillmer/rust,dwillmer/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,dwillmer/rust,graydon/rust,GBGamer/rust,dwillmer/rust,dwillmer/rust,dwillmer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,dwillmer/rust,GBGamer/rust,dwillmer/rust,graydon/rust