This commit adds a bunch of Arxiv tests to demonstrate we can
successfully transform our `Query` type into the Arxiv AST.

It's not fully wired in, `convertQuery` for Arxiv is implemented as
`undefined`.

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.