In #133, Craig added support for a new use of typename, where it applies
to a scalar and means that genqlient should generate a named type, e.g.
`# @genqlient(typename: "MyString")` on a node of type string will
generate and use `type MyString string`. But this gets a bit confusing
if you mix it with `bind`; should
`typename: "MyString", bind: "int32"` generate `type MyString int32`, or
should one override the other, or what? Of course in practice you're
not likely to write that all in one place, but you could via a global
binding, or a `for` directive, and in that case probably it was a
mistake. In #138, we looked at making them work together correctly, but
it added complexity and got even more confusing.
So instead, here, we just ban it; we can always add it back if it proves
useful. (Or, you can make the `typename` win over a global binding by
locally unbinding it via `bind: "-"`.) This required changes in
surprisingly many places; I already knew the directive-validation code
was due for a refactor but that will happen some other day. The tests
show that it works, in any case.
Interestingly, this problem actually could have arisen for a struct
binding already, before #133. But all the same reasons it's confusing
seem to apply, so I just banned it there too. This is technically a
breaking change although I doubt anyone will hit it.
Test plan: make check
