Use of a parameter for contents value
Hi, I can't figure out a way to pass a parameter to the the contents element of a custom type.

The situation is as follows:

```yaml
seq:
  - id: foo
    type: foo_type
  
  - id: bar
    type: bar_type

  foo_type:
    seq:
      - id: name
        contents: foo
  
  bar_type:
    seq:
      - id: name
        contents: bar
```

Since the number of types like foo_type and bar_type grows quickly, I would like to compress this declaration to something like:

```yaml
seq:
  - id: foo
    type: any_type('foo')
  
  - id: bar
    type: any_type('bar')

  any_type:
   params:
     - id: expected_name
       type: ???

    seq:
      - id: name
        contents: ?expected_name?
 
```

In my attempts, the value of contents is treated as a string and not recognized as the parameter of any_type. The type to use in the parameter declaration is also unclear to me.

Is there any way to achieve this?

Thanks !