Rename core/types to core/primitives

Right now, `src/core/types` contains helpers for primitive types: function, string, array, date, enum (honorary primitive), object
This was probably not the best choice of directory naming, since it can cause confusion with type declarations. I'd suggest renaming it to `src/core/primitives`.

The actual change itself is a fairly trivial find/replace, but since it touches many files and is an overall important directory, I wanted to get some perspectives on a) is it even worthwhile and b) bikeshedding "primitives"

@ampproject/wg-performance

Start by reviewing src/core/types and searching the repository for references to that directory. Resolve whether the rename to src/core/primitives and the proposed terminology are accepted before changing files; done means the agreed directory name is used consistently without broken references.

Written by the indexing model from the issue text.