Here we use abstract type & subtyping because it's common traits pattern
in Julia.
We do not need to export these objects because we can use strings for
versions in user-facing functions like:

```julia

function load(str::AbstractString; version::YAMLVersion)
    # ...
end

function load(str::AbstractString; version::AbstractString)
    version == "1.1" ? load(str, version=YAMLV1_1()) :
    version == "1.2" ? load(str, version=YAMLV1_2()) :
    throw(ErrorException())
end

load(str, version="1.1")
```

…o be sorted.