Local records

This can be nice to have for obvious reasons, but it's necessary for some crazier ones (due to lack of #271 and #67).
A very simple example is: imagine you need a (boxed) function that refers to its type in some way (e.g. returns itself). That's not possible normally since that'd be an infinitely long type, but you can write it just fine with records:
```effekt
record Foo(block: () => Foo at {})
```
However, you can't do that if the block has captures:
```effekt
def bar() = region this {
record Foo(block: () => Foo at {this}) // Only supported on the toplevel: record declaration.
}
```

I have a complex use case (which doesn't work currently) with a peculiar scheduler that doesn't use multiple resumptions, but still allows captures.