Ensure entrypoint params are always named

[purcell]

When deployed, some entrypoint parameters are numbered rather than named, which means the contract is a little less self-describing than it should be. Let's see if we can fix it, perhaps by supplying params as typed structs?

See the "Entrypoints" tab here: https://edo.tzstats.com/KT1QYTVYqpnTDR56uY42cNp4GNEGU2oMJeBr

[purcell]

This one is a bit annoying, because it involves declaring the parameter types in a separate file to the endpoints; putting the types in checker.mli isn't enough because they need to be repeated in checker.ml, and if placed in checkerTypes.ml (for example) then the documentation will be fragmented.