Polymorphic top-level Hugrs

[acl-cqc]

Preliminary: note #1733 added the ability to substitute across the nodes/body of the Hugr, rather than just the Types of a Hugr; and #1934 applies this across a subtree copied from a polymorphic funcdefn to a monomorphic callsite.

Here we want a standalone structure of a Hugr+TypeParams, with a validate method (and maybe other Hugr(View/Mut)-like methods), i.e. calling some new Hugr::validate_with_type_params (and Hugr::validate just calls that with the empty list).

Maybe also a method that "instantiates" it to just a Hugr.

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This will(EDIT: might!) be useful for #978; in time for compilation/lowering (by template-extension, rather than type-erasure/interpretation à la Tierkreis/JVM); it offers new Hugr-construction methods (alluded to in #457); it potentially simplifies tests (which could use a type variable instead of pulling in some extension just to give a concrete leaf type - although use of USIZE_T is widespread enough we barely notice).

If we store binders separately from the Hugr nodes, then we need to be careful about additional variables bound by FuncDefn nodes, i.e. can these still see the outer binders? If so, we have a limited return of DeBruijn indices, and will probably need to pass a second &[TypeParam] around validate_subtree (and a first into validate) etc. So perhaps maintain that the outer TypeParams are not visible inside any inner FuncDefns.

[acl-cqc]

So, probably this is some kind of a wrapper around (a Hugr and some TypeParams), that one can validate (-> calls Hugr::validate passing in those TypeParams) and other things.

• Should include an instantiate method that (given TypeArgs correct for all its TypeParams) produces a non-polymorphic Hugr. This is itself a bunch of work. (Partial instantiation probably not required)
• Given No polymorphic closures #904, unclear whether you can have a "polymorphic top level hugr" (with some TypeParams) whose root node is a FuncDefn (binding more TypeParams). If so, then we'll have a limited form of DeBruijn (which was removed in No polymorphic closures #904), and we'll need to pass two sets of TypeParams through validation (the current set, replaced on entry to each FuncDefn, and the outermost "top-level" ones from the wrapper)

[acl-cqc]

This'll also give us a way to do inference separately for subgraphs and thus a better solution to inserting a subgraph than that of test_validate_with_closure from #884

[acl-cqc]

It is just possible that maybe we could treat FuncDefn as the only binder with TypeParams. We would still need a method to apply a substitution across the Hugr (i.e. instantiate a FuncDefn-rooted Hugr to a monomorphic, e.g. DFG-rooted, Hugr)