With a code like
```
thenDo = \x, callback ->
callback x
f = \{} ->
code = 10u16
bf = \{} ->
thenDo code \_ -> bf {}
bf {}
```
The lambda `\_ -> bf {}` must capture `bf`. Previously, this would not
happen correctly, because we assumed that mutually recursive functions
(including singleton recursive functions, like `bf` here) cannot capture
themselves.
Of course, that premise does not hold in general. Instead, we should have
mutually recursive functions capture the closure (haha, get it) of
values captured by all functions constituting the mutual recursion.
Then, any nested closures can capture outer recursive closures' values
appropriately.
This change also means we must update the interface of `Dict.empty` and
`Set.empty` from
```
Dict.empty : Dict k v
```
to
```
Dict.empty : {} -> Dict k v
```
Variables introduced in branch patterns should never be generalized in
the new weakening model. This implements that. The strategy is:
- when we have a let-binding that should be weakened, do not introduce
its bound variables in a new (higher) rank
- instead, introduce them at the current rank, and also solve the
let-binding at the current rank
- if any of those variables should then be generalized relative to the
current rank, they will be so when the current rank is popped and
generalized
We must be careful to ensure that if unifying nested lambda sets
results in disjoint lambdas, that the parent lambda sets are
ultimately treated disjointly as well.
Consider
```
v1: {} -[ foo ({} -[ bar Str ]-> {}) ]-> {}
~ v2: {} -[ foo ({} -[ bar U64 ]-> {}) ]-> {}
```
When considering unification of the nested sets
```
[ bar Str ]
~ [ bar U64 ]
```
we should not unify these sets, even disjointly, because that would
ultimately lead us to unifying
```
v1 ~ v2
=> {} -[ foo ({} -[ bar Str, bar U64 ]-> {}) ] -> {}
```
which is quite wrong - we do not have a lambda `foo` that captures
either `bar captures: Str` or `bar captures: U64`, we have two
different lambdas `foo` that capture different `bars`. The target
unification is
```
v1 ~ v2
=> {} -[ foo ({} -[ bar Str ]-> {}),
foo ({} -[ bar U64 ]-> {}) ] -> {}
```
Closes#4712
