Refactor argument-handling to use a struct (#103)

## Summary:
In this commit I refactor the argument-generation logic to move most of
the code out of the template and into the type-generator.  This logic
predates #51, and I didn't think to update it there, but I think it
benefits from similar treatment, for similar reasons.

Specifically, the main change is to treat variables as another struct
type we can generate, rather than handling them inline as a
`map[string]interface{}`.  Users still pass them the same way, but
instead of putting them into a `map[string]interface{}` and JSONifying
that, we generate a struct and put them there.

This turns out to simplify things quite a lot, because we already have a
lot of code to generate types.  Notably, the omitempty code goes from a
dozen lines to basically two, and fixes a bug (#43) in the process,
because now that we have a struct, `json.Marshal` will do our work for
us! (And, once we have syntax for it (#14), we'll be able to handle
field-level omitempty basically for free.)  More importantly, it will
simplify custom marshalers (#38, forthcoming) significantly, since we do
all that logic at the containing-struct level, but will need to apply it
to arguments.

It does require two breaking changes:

1. For folks implementing the `graphql.Client` API (rather than just
   calling `NewClient`): we now pass them variables as an `interface{}`
   rather than a `map[string]interface{}`.  For most callers, including
   Khan/webapp, this is basically a one-line change to the signature of
   their `MakeRequest`, and it should be a lot more future-proof.
2. genqlient's handling of the `omitempty` option has changed to match
   that of `encoding/json`, in particular it now never considers structs
   "empty".  The difference was never intentional (I just didn't realize
   that behavior of `encoding/json`); arguably our behavior was more
   useful but I think that's outweighed by the value of consistency with
   `encoding/json` as well as the simpler and more correct
   implementation (fixing #43 is actually quite nontrivial otherwise).
   Once we have custom unmarshaler support (#38), users will be able to
   map a zero value to JSON null if they wish, which is mostly if not
   entirely equivalent for GraphQL's purposes.

Issue: https://github.com/Khan/genqlient/issues/38
Issue: https://github.com/Khan/genqlient/issues/43

## 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, ✅ Lint, ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14)

Pull Request URL: https://github.com/Khan/genqlient/pull/103

internal/integration/integration_test.go

@@ -89,6 +89,33 @@ func TestVariables(t *testing.T) {
 	assert.Zero(t, resp.User)
 }
 
+func TestOmitempty(t *testing.T) {
+	_ = `# @genqlient(omitempty: true)
+	query queryWithOmitempty($id: ID) {
+		user(id: $id) { id name luckyNumber }
+	}`
+
+	ctx := context.Background()
+	server := server.RunServer()
+	defer server.Close()
+	client := graphql.NewClient(server.URL, http.DefaultClient)
+
+	resp, err := queryWithOmitempty(ctx, client, "2")
+	require.NoError(t, err)
+
+	assert.Equal(t, "2", resp.User.Id)
+	assert.Equal(t, "Raven", resp.User.Name)
+	assert.Equal(t, -1, resp.User.LuckyNumber)
+
+	// should return default user, not the user with ID ""
+	resp, err = queryWithOmitempty(ctx, client, "")
+	require.NoError(t, err)
+
+	assert.Equal(t, "1", resp.User.Id)
+	assert.Equal(t, "Yours Truly", resp.User.Name)
+	assert.Equal(t, 17, resp.User.LuckyNumber)
+}
+
 func TestInterfaceNoFragments(t *testing.T) {
 	_ = `# @genqlient
 	query queryWithInterfaceNoFragments($id: ID!) {