`Vec1` is used by `RecordType` to ensure that a record have at least 1
field. Then an `InterfaceValue::Record` is ensured to get at least one
value with the type-checking pass.
The `Type::Record` variant now is defined by `RecordType`. In
addition, `InterfaceType` has a new variant: `Record`, that is also
defined by `RecordType`. Encoders and decoders are updated to consider
`RecordType`, which removes code duplication and simplify code.