Type Erasure, Dynamic, Coercions

[croyzor]

Originally raised by @acl-cqc:

• Currently (on main) kind arguments and any value whose type is an earlier (kind) argument are compiled to a Hugr parameter of type unit (i.e. Sum([[]]))
• Instead, arguments whose type is a kind argument should be compiled to have some extension-defined type Dynamic
• The kind arguments themselves might be ok as unit, at least for now - we may need to represent those too eventually (e.g. for dependent tuples), i.e. if any backend needs runtime type information (?)
• We'll also need to fix the type errors in Hugr validation - any time a value of known type is converted to a Dynamic, or vice versa, and also any time a graph/higher-order-function-value (say Int->Int passed into a Dynamic->Dynamic), we'll need to add a magic coercion op