Issue not-yet-existent types to grammars

[THinnerichs]

I build a grammar, and add an additional rule to it.

julia> grammar
1: Number = _
2: Number = Number + Number
3: Number = Number * Number
4: ntString = if ntBool
    ntString
else
    ntString
end

As one can see, ntBool is not a type of grammar. Hence, all_types in add_rule! doesn't hole ntBool. Hence the rule also only has two children, not three.

This should not fail during grammar construction, but whenever I try to search over it.