rustdoc: Implement `is_primitive` in terms of `primitive_type()`
rustdoc: Implement `is_primitive` in terms of `primitive_type()`

Previously, they disagreed about what types were primitives.

This also fixes a bug where rustdoc would mark any type with a reference
as a primitive.

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