executeField shouldn't assume that a selection has only one field with a
given name, but it should take the first field. The underlying cause is
a wrong pattern, which (because of the laziness) is executed only if the
field has arguments.

The functions generating errors in the executor should be changed anyway
when we provide better error messages from the executor, with the error
location and response path. So public definitions of these functions are
deprecated now and they are replaced by more generic functions in the
executor code.

I found some issues with directive definitions:

- I couldn't use `on FIELD_DEFINITION`, I believe because `FIELD` was parsed
  first in `executableDirectiveLocation`. I've combined both
  `executableDirectiveLocation` and `typetypeSystemDirectiveLocation` into one
  function which can reorder them to ensure every directive location gets a fair
  chance at parsing.

Not actually to do with directives, some literals weren't being parsed
correctly.

- The GraphQL spec defines list to be `[]` or `[Value]`, but empty literal lists
  weren't being parsed correctly because of using `some` instead of `many`.

- The GraphQL spec defines objects to be `{}` or `{Name: Value}`, but empty
  literal objects had the same issue.