Reject operation or argument names that are Go keywords (#195)

GraphQL is pretty restrictive about its identifiers, so for the most
part we can and do safely use GraphQL identifiers in the Go we generate
with attention only to conflicts with other such identifiers. But we do
need to check one thing, which is that the identifier isn't a Go
keyword. (If it is, the generated code will almost certainly fail to
compile, but often with a confusing error message.) In this commit I add
such checks.

The most likely place to run into trouble here is argument names, which
are often one word and are used as-is.  Operation names, if unexported,
can also be keywords, although in practice they're usually multiword.
Field names are always exported, thus safe.  Generated type names are
always prefixed, camel-cased, with the operation name, so they always
contain an uppercase letter (even if the operation name is lowercase),
but type-names specified by `typename` may collide, so we check those.
In theory we could check type-names specified by `bind`, but these must
be defined by the user, so their code will already fail to compile, so I
didn't bother.  I think that's all the places to consider, although it's
hard to be sure.  In summary, we check argument names, operation names,
and user-specified type names.

Test plan: make check

generate/testdata/errors/KeywordArgumentName.graphql

@@ -0,0 +1,3 @@
+query KeywordArgumentName($type: String) {
+  f(type: $type)
+}

generate/testdata/errors/KeywordArgumentName.schema.graphql

@@ -0,0 +1 @@
+type Query { f(type: String): String }

generate/testdata/errors/KeywordOperationName.graphql

@@ -0,0 +1 @@
+query type { f }

generate/testdata/errors/KeywordOperationName.schema.graphql

@@ -0,0 +1 @@
+type Query { f: String }

generate/testdata/errors/KeywordTypeName.graphql

@@ -0,0 +1,6 @@
+query KeywordTypeName {
+  # @genqlient(typename: "type")
+  f {
+    g
+  }
+}

generate/testdata/errors/KeywordTypeName.schema.graphql

@@ -0,0 +1,2 @@
+type Query { f: T }
+type T { g: String }

generate/testdata/snapshots/TestGenerateErrors-KeywordArgumentName-graphql

@@ -0,0 +1 @@
+testdata/errors/KeywordArgumentName.graphql:1: variable name must not be a go keyword

generate/testdata/snapshots/TestGenerateErrors-KeywordOperationName-graphql

@@ -0,0 +1 @@
+testdata/errors/KeywordOperationName.graphql:1: operation name must not be a go keyword

generate/testdata/snapshots/TestGenerateErrors-KeywordTypeName-graphql

@@ -0,0 +1 @@
+testdata/errors/KeywordTypeName.schema.graphql:1: typename option must not be a go keyword