RustWrapper: remove some uses of AttrBuilder
RustWrapper: remove some uses of AttrBuilder

Turns out we can also use Attribute::get*() methods here, and avoid the
AttrBuilder and an extra helper method here.

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