This commits changes the `PhyloData` data structure to
include a proper `GraphData` in the `pd_data` field. The
`GraphData` is almost complete, but the `objects` field needs
to be further refined from a generic JSON `Value` to proper
Haskell types.

This reverts commit a444cb30.

This reverts commit 22b14f56.

This reverts commit d04cf03b, reversing
changes made to e213a4bf.

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.