When we have a tag union type, outside of special cases in the middle of
unification, its extension type should either be (1) the closed tag
union or (2) a flex/rigid var. This adds an assertion for that.
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
This materially improves performance for programs that are
recursion-heavy (as most Roc programs will likely be).
```
$ hyperfine '/tmp/roc-old check examples/cli/cli-platform/Arg.roc' '/tmp/roc-new check examples/cli/cli-platform/Arg.roc' --warmup 10
Benchmark 1: /tmp/roc-old check examples/cli/cli-platform/Arg.roc
Time (mean ± σ): 53.8 ms ± 1.3 ms [User: 87.3 ms, System: 10.8 ms]
Range (min … max): 52.2 ms … 60.4 ms 51 runs
Benchmark 2: /tmp/roc-new check examples/cli/cli-platform/Arg.roc
Time (mean ± σ): 45.0 ms ± 1.6 ms [User: 59.4 ms, System: 11.3 ms]
Range (min … max): 42.6 ms … 49.8 ms 60 runs
Summary
'/tmp/roc-new check examples/cli/cli-platform/Arg.roc' ran
1.20 ± 0.05 times faster than '/tmp/roc-old check examples/cli/cli-platform/Arg.roc'
```
The time spent in `occurs` during checking for `Arg` drops from 50% to 23%.
We're now reaching the steady state we want to be closert to - when a
type is translated to a variable, emplace the variable we created for it
in the type index, so that types are never converted again!
