Use memoization in file-mapping to improve performance.
Use of the `:file-mapping` to `check-ns-info` now increases run time by a good factor of 5. By memoizing the conversion of types into strings, we can reduce this to a factor of 2. This will also reduce memory consumption, as multiple entries for the same symbol will now contain references to the same string, rather than copies.
Moving to a different approach, declined.
I'm thinking of providing the user an atom instead of a map which will be updated when necessary.