Fix broken behavior for several options on the same node (#105)

## Summary:
Previously, we actually allowed you to put several genqlient directives
on the same node, but the semantics were undocumented (and somewhat
confusing, when it comes to `typename`).  In order to support directives
on input options, we're actually going to be encouraging this usage (see
notes in #14), so it's time to fix it.  To avoid confusion, I just had
conflicting directives be an error, rather than defining which one
"wins".  The same applies to specifying the same option several
times in one directive.

I also fixed two small bugs:
- `typename` on an operation would incorrectly cascade down to
  all input types in a query (causing conflicts).
- directive parse errors had useless positions, now they're correct

## Test plan:
make check


Author: benjaminjkraft

Reviewers: StevenACoffman, dnerdy, aberkan, jvoll, mahtabsabet, MiguelCastillo

Required Reviewers: 

Approved By: StevenACoffman, dnerdy

Checks: ⌛ Test (1.17), ⌛ Test (1.16), ⌛ Test (1.15), ⌛ Test (1.14), ⌛ Lint, ⌛ Test (1.17), ⌛ Test (1.16), ⌛ Test (1.15), ⌛ Test (1.14), ⌛ Lint

Pull Request URL: https://github.com/Khan/genqlient/pull/105

docs/CHANGELOG.md

@@ -28,6 +28,7 @@ When releasing a new version:
 ### New features:
 
 - The new `bindings.marshaler` and `bindings.unmarshaler` options in `genqlient.yaml` allow binding to a type without using its standard JSON serialization; see the [documentation](genqlient.yaml) for details.
+- Multiple genqlient directives may now be applied to the same node, as long as they don't conflict; see the [directive documentation](genqlient_directive.graphql) for details.
 
 ### Bug fixes:
 

docs/genqlient_directive.graphql

@@ -21,15 +21,25 @@
 #  # @genqlient(n: "c")
 #  query MyQuery(arg1: String,
 #    # @genqlient(n: "d")
-#    arg2: String, arg3: String,
+#    arg2: String, arg3: MyInput,
 #    arg4: String,
 #  ) {
 #    # @genqlient(n: "e")
 #    field1, field2
-#    field3
+#    # @genqlient(n: "f")
+#    field3 {
+#      field4
+#    }
 #  }
 # the directive "a" is ignored, "b" and "c" apply to all relevant nodes in the
-# query, "d" applies to arg2 and arg3, and "e" applies to field1 and field2.
+# query, "d" applies to arg2 and arg3, "e" applies to field1 and field2, and
+# "f" applies to field3.
+#
+# Except as noted below, directives on nodes take precedence over ones on the
+# entire query (so "d", "e", and "f" take precedence over "b" and "c"), and
+# multiple directives on the same node ("b" and "c") must not conflict.  Note
+# that directives on nodes do *not* apply to their "children", so "d" does not
+# apply to the fields of MyInput, and "f" does not apply to field4.
 directive genqlient(
 
   # If set, this argument will be omitted if it has an empty value, defined
@@ -125,7 +135,9 @@ directive genqlient(
   # down to all child fields (which would cause conflicts).
   typename: String
 
-) on
+# Multiple genqlient directives are allowed in the same location, as long as
+# they don't have conflicting options.
+) repeatable on
   # genqlient directives can go almost anywhere, although some options are only
   # applicable in certain locations as described above.
   | QUERY