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.

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.

The old function, addErrMsg, takes only a string with an error
description, but more information is required for the execution errors:
locations and path. addErrMsg should be deprecated after the switching
to the new addError.