Built-in ASCII/BCD integer type
I known, it is possible to use 
```yaml
types:
  int:
    params:
      - id: len
        type: u1
    seq:
      - id: as_str
        size: len
        type: str
        encoding: ASCII
    instances:
      as_int:
        value: as_str.to_i
```
but is is ugly. Many financial protocols (and, I think, other protocols) use ASCII encoded numbers, so it might useful to have built-in type to deal with them