Rollup merge of #88859 - RalfJung:size-helpers, r=oli-obk
Rollup merge of #88859 - RalfJung:size-helpers, r=oli-obk

interpreter PointerArithmetic: use new Size helper methods

aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust