Allow templating string for inherited and generics

[gabotechs]

For generics there could be something like

"""
This is a $T
"""
type Generic<T> {
    " This is a description for $T.foo"
    foo: T
}

For inheritance something like

type _Common {
   " field $PARENT.foo "
    foo: String!
}

[gabotechs]

implemented in #32, but it is a bit more complex than in the example above: this is a working example:

"I am ${{ block.name }}, and my arguments are ${{ variables.T }} and ${{ variables.U }}"
type Generic<T U> {
    "I am ${{ variables.T }}"
    t: T
    "I am ${{ variables.U }}"
    u: U
}

type Concrete = Generic<String, Int>

will compile to

"I am Concrete, and my arguments are String and Int"
type Concrete {
    "I am String"
    t: String
    "I am Int"
    u: Int
}