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 patch provides errors for defs that are used only in
possibly-mutual recursion, and are not reachable outside of their
recursive closures. For example:
```
test_report!(
mutual_recursion_not_reached_nested,
indoc!(
r#"
app "test" provides [main] to "./platform"
main =
f = \{} -> if Bool.true then "" else g {}
g = \{} -> if Bool.true then "" else f {}
""
"#
),
@r###"
── DEFINITIONs ONLY USED IN RECURSION ──────────────────── /code/proj/Main.roc ─
These 2 definitions are only used in mutual recursion with themselves:
4│> f = \{} -> if Bool.true then "" else g {}
5│> g = \{} -> if Bool.true then "" else f {}
If you don't intend to use or export any of them, they should all be
removed!
"###
);
```
Programs like
```
after : ({} -> a), ({} -> b) -> ({} -> b)
fx = after (\{} -> {}) \{} -> if Bool.true then fx {} else {}
```
are legal because they always decay to functions, even if they may not
look like functions syntactically. Rather than using a syntactic check
to check for illegally-recursive functions, we should only perform such
checks after we know the types of values.
Closes#4291
