Address review comments
Address review comments

- Move static variables into the innermost scope in which they are used
- Clean up comments
- Remove external_providers; rename local_providers -> providers

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