Add Haskell types which map statically to tuples

As a sort of ORM-lite, it would be handy to be able to push lots of db client type errors to compile time from run time. A good example of how to accomplish this is via https://github.com/Gabriel439/Haskell-Bears-Library.

However, the approach there makes several assumptions about the relational algebra which do not necessarily hold. For example, all relvars have exactly one key; this the deprecated notion of a "primary" key.

Some Template Haskell may be involved here.