Convert Sort Expression to SQL

Converts 'sort format' to 'SQL format'. E.g.:

• -id -> id DESC NULLS LAST
• id -> id ASC NULLS LAST
• id- -> id ASC NULLS FIRST
• -aired,id -> aired DESC NULLS LAST, id ASC NULLS LAST

Example

#![allow(unstable)]
# use sort_str_to_sql::{sort_str_to_sql};
# use std::borrow::ToOwned;
assert!(
  sort_str_to_sql("-id") ==
  Some("id DESC NULLS LAST".to_owned())
);
assert!(
  sort_str_to_sql("-id,+aired-") ==
  Some("id DESC NULLS LAST, aired ASC NULLS FIRST".to_owned())
);

(See tests for more examples.)