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.

It doesn't add anything over just `State NgramsTableMap ()`, if not
obscuring what the single functions should actually be doing.

This reverts commit b7e406b1.

Unique index is necessary when one wants to refresh the view
concurrently.

This doesn't block the whole view for the (long) period of refreshing
it.