This reverts commit d04cf03b, reversing
changes made to e213a4bf.

This commit simplifies the `ExternalAPIs` type, which is now a pure
enumeration, moving the lookup of the Pubmed key at the usage sites.
This simplified the code quite a bit, and made some otherwise-effectful
functions pure.

This is safer than type Limit = Int

Read in backend works now, insert and upsert not yet.