interpreter PointerArithmetic: use new Size helper methods
interpreter PointerArithmetic: use new Size helper methods

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