Unify Mixin and DR behaviour (and related Functions stuff)

As dissucussed in #2767, current DR and Mixin call result (visibility of their internal entities in the caller scope) is somewhat inconsistent. In particular, mixin calls exposes both internal mixins and variables, but DR calls keep its variables hidden:

Mixin:

.my-red-theme() {
    .button() {
        color: red;  
    }

    @color: red;
}

.btn {
    .my-red-theme();
    .button();            // OK
    border-color: @color; // OK
}

DR:

@my-red-theme: {
    .button() {
        color: red;  
    }

    @color: red;
};

.btn {
    @my-red-theme();
    .button();            // OK
    border-color: @color; // Error
}

The proposal is to match both (per #2767 by exposing DR variables too).

The exposed variables ("callee variables") have the lowest priority (i.e. never override caller's local or parent scope variables), thus the change will not break any existing Less code.

Edit: My mistake above: callee variables do override caller's parent (incl. global) variables, thus the change is breaking for snippets like:

@my-red-theme: {
    @color: red;
};

@color: blue;

.btn {
    @my-red-theme();
    color: @color; // blue -> red
}

Start by reading the discussion in issue #2767 and comparing the Mixin and DR examples in this issue. Trace how DR and mixin calls expose entities in the caller scope, including the related Functions behavior. Done means the visibility rules are consistent and the documented breaking case is covered.