Prevent panic when parsing record type with field path #1325

Nickel does not support record types with field paths. i.e. while this is a valid type: ``` { x : { y : String } } ``` this is not: ``` { x.y : String } ``` However, at some point rather than raising the appropriate error here, we started panicking. This commit fixes that by checking that a field's path only contains a single element before deciding whether to allow it in a record type.