Retain the bytes used to construct a file

Later on this will be useful when printing diagnostics with source code
snippets as context.

zcl/json/public.go

@@ -33,6 +33,7 @@ func Parse(src []byte, filename string) (*zcl.File, zcl.Diagnostics) {
 			Body: &body{
 				obj: rootNode.(*objectVal),
 			},
+			Bytes: src,
 		}
 	}
 	return file, diags

zcl/structure.go

@@ -6,7 +6,8 @@ import (
 
 // File is the top-level node that results from parsing a ZCL file.
 type File struct {
-	Body Body
+	Body  Body
+	Bytes []byte
 }
 
 // Block represents a nested block within a Body.