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

graphql/client.go

@@ -24,9 +24,10 @@ type Client interface {
 	// context.Background().
 	//
 	// query is the literal string representing the GraphQL query, e.g.
-	// `query myQuery { myField }`.  variables contains the GraphQL variables
-	// to be sent along with the query, or may be nil if there are none.
-	// Typically, GraphQL APIs will accept a JSON payload of the form
+	// `query myQuery { myField }`.  variables contains a JSON-marshalable
+	// value containing the variables to be sent along with the query,
+	// or may be nil if there are none.  Typically, GraphQL APIs will
+	// accept a JSON payload of the form
 	//	{"query": "query myQuery { ... }", "variables": {...}}`
 	// but MakeRequest may use some other transport, handle extensions, or set
 	// other parameters, if it wishes.
@@ -41,8 +42,7 @@ type Client interface {
 		ctx context.Context,
 		opName string,
 		query string,
-		retval interface{},
-		variables map[string]interface{},
+		input, retval interface{},
 	) error
 }
 
@@ -69,8 +69,8 @@ func NewClient(endpoint string, httpClient *http.Client) Client {
 }
 
 type payload struct {
-	Query     string                 `json:"query"`
-	Variables map[string]interface{} `json:"variables,omitempty"`
+	Query     string      `json:"query"`
+	Variables interface{} `json:"variables,omitempty"`
 	// OpName is only required if there are multiple queries in the document,
 	// but we set it unconditionally, because that's easier.
 	OpName string `json:"operationName"`
@@ -81,7 +81,7 @@ type response struct {
 	Errors gqlerror.List `json:"errors"`
 }
 
-func (c *client) MakeRequest(ctx context.Context, opName string, query string, retval interface{}, variables map[string]interface{}) error {
+func (c *client) MakeRequest(ctx context.Context, opName string, query string, retval interface{}, variables interface{}) error {
 	body, err := json.Marshal(payload{
 		Query:     query,
 		Variables: variables,