Implicit `cases` for T(L)V structures
In TLV structs we usually define a enum for a tag, and very often this enum vslue  atches 1-to-1 to the types parsed in value. It is proposed to allow making that `switch-on` implicit:
when a enum value with name `a` is matched, the type with the same name should be selected.
 
```yaml
seq:
  - id: a
    type: u1
  - id: tag
    type: u1
    enum: tag
  - id: value
    type:
      switch-on: tag
      cases-implicit:
        name: "{}_qwerty(a)"  # must contain exactly one substitution. can contain params
      cases: # explicitly generated cases take precedence
        _: asdf
types:
  c_asdf:
    ...
  a_qwerty:
    ...
  b_qwerty:
    ...
enums:
  1: a
  2: b
```

This proposal intended to solve 2 violations of the DRY principle:
1. devs have to write and read large sheets in cases, whicha re trivial
2. the sheets duplicate the same text.