Add a type parameter to TypeF for contracts

[yannham]

The TypeF recursion scheme is parametrized by a number of (Rust) types: the type of Nickel types, the type of enum rows and the type of record rows. There is one more Rust type that can be stored in a TypeF: a contract. It's currently hardcoded to be a RichTerm, which is fine because we never want to use another Rust type to store something else

With the development of the experimental bytecode interpreter, we are changing the representation of terms. Because we want to keep mailine Nickel operational during the transition, we have to copy paste all the typ module somewhere else, replacing RichTerm with the type of the new AST.

There is an alternative: as TypeF is already parametrized by a bunch of things, why not also parametrize the type of what's stored inside TypeF::Contract? This adds a bit more complexity, and requires an additional closure when mapping on or recursing into a Type. But the price doesn't seem too high, and we can reuse the same implementation for both mainline Nickel and bytecode Nickel.

There is an unexpected benefit: in fact, we do store something else than a RichTerm! During typechecking, we need to store an additional environment alongside a contract in order to properly compute contract equality. This led to having an additional constructor GenericUnifType::Contract, that must be handled separately from concrete types. When converting from a Type to a UnifType, the node TypeF::Flat is converted toGenericUnifType::Contract, and all typechecking functions then assume the invariant that they will never encounter a TypeF::Flat. But the Rust type system can't guarantee it. There's also a bespoke runtime error for that (although we're pretty sure it can't happen), a bunch of debug_assert, etc.

We can thus kill two birds with one stone: by parametrizing TypeF, we can now instantiate it with (RichTerm, Environment) for unification types. No need for runtime invariants or ad-hoc treatment anymore.

Doing so, we also rename TypeF::Flat to TypeF::Contract, the former being a historical but bad name.

[github-actions[bot]]

Bencher Report

Branch	2078/merge
Testbed	ubuntu-latest

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Benchmark	Latency	nanoseconds (ns)
fibonacci 10	📈 view plot ⚠️ NO THRESHOLD	486,400.00
foldl arrays 50	📈 view plot ⚠️ NO THRESHOLD	1,697,700.00
foldl arrays 500	📈 view plot ⚠️ NO THRESHOLD	6,655,300.00
foldr strings 50	📈 view plot ⚠️ NO THRESHOLD	7,044,900.00
foldr strings 500	📈 view plot ⚠️ NO THRESHOLD	62,522,000.00
generate normal 250	📈 view plot ⚠️ NO THRESHOLD	44,732,000.00
generate normal 50	📈 view plot ⚠️ NO THRESHOLD	2,083,700.00
generate normal unchecked 1000	📈 view plot ⚠️ NO THRESHOLD	3,367,300.00
generate normal unchecked 200	📈 view plot ⚠️ NO THRESHOLD	756,840.00
pidigits 100	📈 view plot ⚠️ NO THRESHOLD	3,217,500.00
pipe normal 20	📈 view plot ⚠️ NO THRESHOLD	1,509,600.00
pipe normal 200	📈 view plot ⚠️ NO THRESHOLD	10,409,000.00
product 30	📈 view plot ⚠️ NO THRESHOLD	825,310.00
scalar 10	📈 view plot ⚠️ NO THRESHOLD	1,519,200.00
sum 30	📈 view plot ⚠️ NO THRESHOLD	823,440.00

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