Also remove stack.yaml since it isn't used anymore. and adding libraries
to the snapshots doesn't seem to be as easy as I hoped.

The intermediate representation was further modified so that the
operation definitions contain location information. Probably I should
introduce a data type that generalizes fields and operations, so it
contains object type, location and the selection set, so the functions
don't accept so many arguments.

The executor still doesn't give an error per argument, but a single
error per field with locations for all arguments.
If a non-null argument isn't specified, only the error location of the
field is given. If some arguments cannot be coerced, only the locations
of these arguments are given, non-null arguments are ignored. This
should still be improved, so the executor returns all errors at once.
The transformation tree is changed, so that argument map contains
locations of the arguments (but not the locations of the argument values
yet).

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.