Omit does not work on intersections

```
[Node] ~/backend/node_modules/simple-runtypes/src/omit.ts:15
[Node]     throw new RuntypeUsageError(`expected a record runtype`)
[Node]           ^
[Node] Error: expected a record runtype
[Node]     at Object.omit (~/backend/node_modules/simple-runtypes/src/omit.ts:15:11)
```
