Circular reference causes memory leak / crashes

Take this scenario

file1.yaml
```
field:
$ref: './file2.yaml#/components/schemas/Something'
```
file2.yaml
```
AnotherThing:
$ref: './file1.yaml#/components/schemas/SomethingElse'
```

Currently apisprout will spike in memory usage or crash in such a scenario, even though references are resolvable.

Start by reproducing the issue with the two YAML files shown in the report, then trace apisprout's OpenAPI reference-resolution entry point to see how the cross-file cycle is handled. Done means these resolvable circular references no longer cause unbounded memory use or a crash.

Written by the indexing model from the issue text.