refactor!(hugrv2): WIP+RFC: Unify Type and Term

[acl-cqc]

Follows #2784; closes #2341

• Abandon the Type/TypeRV distinction. Yes, this loses us some Rust static type checking. But RowVars were always a hack, in that they looked like one entry in a list, but were any number of entries. Instead now we have variables that range over lists (i.e. are not members of lists) - there is (already) a Term::ListConcat which is a much nicer/more-principled solution.
  • See 68fba45 for a way of bringing back some of the ease-of-use/compactness of the RV system using the checking we have implemented in Rust; however, the "improvement" in the user code (/the pain of having to explicitly Term::new_list_concat) was not that great so I have reverted...thoughts?
• Define type Type/TypeRV = Term; remove Term::Runtime(Type) and add new Term constructors corresponding to the old ones in Type: Term::RuntimeFunction, Term::RuntimeSum and Term::RuntimeExtension.
  • Note that the next PR probably makes that just ::Extension, i.e. able to produce things that are not just runtime types....
• I removed Aliases; I mean I could add them but they are vestigial and not really working anyway (Overhaul or scrap aliases #2558). Or perhaps we should add them in and support properly now we have linking?
• TypeRow now stores Terms (of course)
• Signature stores a TypeRow of fixed arity - each element must be a runtime type (exactly one, so no RVs); FuncValueType stores a Term (an instance of Term::ListType(Term::RuntimeType)) which could thus contain variables ranging over lists (RVs).

This is mostly pain, the bigger gains are expected to come in later PRs (#2296 and #2756, eventually allowing "inspectable" custom constants with efficient serialization). But perhaps removing the RV hack is a win.

It's a draft, things are not all working yet....

• Everything serde-related breaks; I've disabled roundtrip+schema-check in validate, but there are still many other tests. I think is a no-go, estimate that producing backward-compatible serde output will be....waaaaay too much effort; we plan to deprecate serde anyway, so this might be for after it's gone. But, will we need to keep serde for serializing Extensions (not Hugrs)?
• Proptesting is broken...wondering whether we need to do something like test: proptest Type validation and substitution #1744 i.e. specifying the type of the Term to be produced by Arbitrary.
• Also there is some hugr-llvm snapshot weirdness

TODOs/unresolved questions:

• Is it time to retire old aliases TypeArg, TypeParam and the new TypeRV, Type, TypeRowRV ?
• What about TypeRow? Rename to TermRow? Or follow suggestion in previous PR and remove it?
• Some naming is a bit dodgy....probably quite a lot....
• I rather feel that we should rename all the Term::XXXType to XXXKind, and all Term::RuntimeYYY to ::YYYType (so Term::SumType, Term::ExtensionType, Term::FunctionType). Using the word "type" to mean types at the static/Term level rather than runtime (wire) types seems confusing.

• The FuncValueType/Signature/SumType methods new, try_new and new_unchecked were a big help in debugging, getting back some of what was lost by the unification of Type-Term and RV-NoRV. I suspect we should extend this to all OpType variants - ExtensionOp, Call, and LoadFunction have always done these checks, so we could add to Input, Output, containers, etc. etc. too (?)
• hugr-py needs updating too

BREAKING CHANGE: Term gains RuntimeFunction, RuntimeSum and RuntimeExtension variants; Type is now an alias for Term. Use check_term_type(tm, &TypeBound::Linear.into()) to check that tm: &Term represents a runtime type. No more aliases.

[hugrbot]

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