Add support for binding with a custom marshal/unmarshal function (#104)

## Summary:
This is useful if you want to bind to a type you don't control (or use
for other things) but need different serialization than its default.
This is a feature gqlgen has and we've found it very useful.  For
example, in webapp we want to bind `DateTime` to `time.Time`, but its
default serialization is not compatible with Python, so currently we
have to bind to a wrapper type and cast all over the place, which is
exactly the sort of boilerplate genqlient is supposed to avoid.

For unmarshaling, the implementation basically just follows the existing
support for abstract types; instead of calling our own generated
helper, we now call your specified function.  This required some
refactoring to abstract the handling of custom unmarshalers generally
from abstract types specifically, and to wire in not only the
unmarshaler-name but also the `generator` (in order to compute the right
import alias).

For marshaling, I had to implement all that stuff over again; it's
mostly parallel to unmarshaling (and I made a few minor changes to
unmarshaling to make the two more parallel).  Luckily, after #103 I at
least only had to do it once, rather than implementing the same
functionality for arguments and for input-type fields.  It was still
quite a bit of code; I didn't try to be quite as completionist about the
tests as with unmarshal but still had to add a few.

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

## Test plan:
make check


Author: benjaminjkraft

Reviewers: StevenACoffman, dnerdy, benjaminjkraft, 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/104

generate/testdata/snapshots/TestGenerate-InputObject.graphql-InputObject.graphql.go

@@ -3,6 +3,10 @@ package test
 // Code generated by github.com/Khan/genqlient, DO NOT EDIT.
 
 import (
+	"encoding/json"
+	"fmt"
+	"time"
+
 	"github.com/Khan/genqlient/graphql"
 	"github.com/Khan/genqlient/internal/testutil"
 )
@@ -54,6 +58,32 @@ type UserQueryInput struct {
 	Role       Role             `json:"role"`
 	Names      []string         `json:"names"`
 	HasPokemon testutil.Pokemon `json:"hasPokemon"`
+	Birthdate  time.Time        `json:"-"`
+}
+
+func (v *UserQueryInput) MarshalJSON() ([]byte, error) {
+
+	var fullObject struct {
+		*UserQueryInput
+		Birthdate json.RawMessage `json:"birthdate"`
+		graphql.NoMarshalJSON
+	}
+	fullObject.UserQueryInput = v
+
+	{
+
+		dst := &fullObject.Birthdate
+		src := v.Birthdate
+		var err error
+		*dst, err = testutil.MarshalDate(
+			&src)
+		if err != nil {
+			return nil, fmt.Errorf(
+				"Unable to marshal UserQueryInput.Birthdate: %w", err)
+		}
+	}
+
+	return json.Marshal(&fullObject)
 }
 
 // __InputObjectQueryInput is used internally by genqlient