check_descriptor() does not yet include all types in the Value enum

[Dhghomon]

Looks like check_descriptor() is still unfinished in places, such as enums (also ranges and some others). Will start with a PR for enum and once merged do the rest.

https://github.com/edgedb/edgedb-rust/blob/master/edgedb-protocol/src/query_arg.rs#L242

Update: Partially completed with #288