Separate multiple inherited interfaces with &

This replaces:

```graphql
type Foo implements Bar, Baz { field: Type }
```

With:

```graphql
type Foo implements Bar & Baz { field: Type }
```

This is more consistent with other trailing lists of values which
either have an explicit separator (union members) or are prefixed
with a sigil (directives). This avoids parse ambiguity in the
case of an omitted field set, illustrated by
[github.com/graphql/graphql-js#1166](graphql/graphql-js#1166).

For now, the old method of declaration remains valid.

References:
- graphql/graphql-js#1169
- graphql/graphql-spec#90
- graphql/graphql-spec@32b55ed

internal/common/lexer.go

@@ -20,7 +20,7 @@ type Ident struct {
 	Loc  errors.Location
 }
 
-func New(sc *scanner.Scanner) *Lexer {
+func NewLexer(sc *scanner.Scanner) *Lexer {
 	l := &Lexer{sc: sc}
 	l.Consume()
 	return l

internal/query/query.go

@@ -100,7 +100,7 @@ func Parse(queryString string) (*Document, *errors.QueryError) {
 	}
 	sc.Init(strings.NewReader(queryString))
 
-	l := common.New(sc)
+	l := common.NewLexer(sc)
 	var doc *Document
 	err := l.CatchSyntaxError(func() {
 		doc = parseDocument(l)

internal/schema/schema.go

@@ -154,13 +154,14 @@ func New() *Schema {
 	return s
 }
 
+// Parse the schema string.
 func (s *Schema) Parse(schemaString string) error {
 	sc := &scanner.Scanner{
 		Mode: scanner.ScanIdents | scanner.ScanInts | scanner.ScanFloats | scanner.ScanStrings,
 	}
 	sc.Init(strings.NewReader(schemaString))
 
-	l := common.New(sc)
+	l := common.NewLexer(sc)
 	err := l.CatchSyntaxError(func() {
 		parseSchema(s, l)
 	})
@@ -316,7 +317,7 @@ func parseSchema(s *Schema, l *common.Lexer) {
 			}
 			l.ConsumeToken('}')
 		case "type":
-			obj := parseObjectDecl(l)
+			obj := parseObjectDeclaration(l)
 			obj.Desc = desc
 			s.Types[obj.Name] = obj
 			s.objects = append(s.objects, obj)
@@ -351,22 +352,26 @@ func parseSchema(s *Schema, l *common.Lexer) {
 	}
 }
 
-func parseObjectDecl(l *common.Lexer) *Object {
-	o := &Object{}
-	o.Name = l.ConsumeIdent()
+func parseObjectDeclaration(l *common.Lexer) *Object {
+	object := &Object{Name: l.ConsumeIdent()}
+
 	if l.Peek() == scanner.Ident {
 		l.ConsumeKeyword("implements")
-		for {
-			o.interfaceNames = append(o.interfaceNames, l.ConsumeIdent())
-			if l.Peek() == '{' {
-				break
+
+		for l.Peek() != '{' {
+			object.interfaceNames = append(object.interfaceNames, l.ConsumeIdent())
+
+			if l.Peek() == '&' {
+				l.ConsumeToken('&')
 			}
 		}
 	}
+
 	l.ConsumeToken('{')
-	o.Fields = parseFields(l)
+	object.Fields = parseFields(l)
 	l.ConsumeToken('}')
-	return o
+
+	return object
 }
 
 func parseInterfaceDecl(l *common.Lexer) *Interface {

internal/schema/schema_test.go

@@ -0,0 +1,71 @@
+package schema
+
+import (
+	"strings"
+	"testing"
+	"text/scanner"
+
+	"github.com/graph-gophers/graphql-go/internal/common"
+)
+
+type testCase struct {
+	description string
+	declaration string
+	expected    *Object
+}
+
+func TestParseObjectDeclaration(t *testing.T) {
+	tests := []testCase{
+		{
+			"allows '&' separator",
+			"Alien implements Being & Intelligent { name: String, iq: Int }",
+			&Object{
+				Name:           "Alien",
+				interfaceNames: []string{"Being", "Intelligent"},
+			},
+		},
+		{
+			"allows legacy ',' separator",
+			"Alien implements Being, Intelligent { name: String, iq: Int }",
+			&Object{
+				Name:           "Alien",
+				interfaceNames: []string{"Being", "Intelligent"},
+			},
+		},
+	}
+
+	setup := func(schema string) *common.Lexer {
+		sc := &scanner.Scanner{
+			Mode: scanner.ScanIdents | scanner.ScanInts | scanner.ScanFloats | scanner.ScanStrings,
+		}
+		sc.Init(strings.NewReader(schema))
+		return common.NewLexer(sc)
+	}
+
+	for _, test := range tests {
+		t.Run(test.description, func(t *testing.T) {
+			lex := setup(test.declaration)
+			var actual *Object
+
+			parse := func() { actual = parseObjectDeclaration(lex) }
+			if err := lex.CatchSyntaxError(parse); err != nil {
+				t.Fatal(err)
+			}
+
+			if test.expected.Name != actual.Name {
+				t.Errorf("wrong object name: want %q, got %q", test.expected.Name, actual.Name)
+			}
+
+			if len(test.expected.interfaceNames) != len(actual.interfaceNames) {
+				t.Fatalf("wrong number of interface names: want %s, got %s", test.expected.interfaceNames, actual.interfaceNames)
+			}
+
+			for i, expectedName := range test.expected.interfaceNames {
+				actualName := actual.interfaceNames[i]
+				if expectedName != actualName {
+					t.Errorf("wrong interface name: want %q, got %q", expectedName, actualName)
+				}
+			}
+		})
+	}
+}