Allow `contents` key be just a one byte
In protocol, on which I currently working, frequently used delimiter bytes, such as FS (0x1C), GS (0x1D), and so on. Right now I'm forced to describe they as
```yaml
seq:
  - contents: [0x1C]
```
which translates to slightly non-optimal code, which involves array creation:
```java
// Java example
this._unnamed1 = this._io.ensureFixedContents(new byte[] { 28 });
```
I think, that such delimiter bytes are widely used in many protocols, so it will be great, if I could write
```yaml
seq:
  - contents: 0x1C
```
and get
```java
// Java example
this._io.ensureFixedContents(28);
```
(example also include applied fix for #664)