Support multiple transformations in one diff

[abhinav]

Diffs in a patch can currently transform only one entity at a time.

That is,

@@
@@
 type Foo struct {
-  bar string
 }
 
@@
@@
 type Bar struct {
-  foo string
 }

We special case statements by allowing statement blocks, but nothing else. Where appropriate -- specially for type and func declarations, we should support multiple entities in the same patch.

@@
@@
 type Foo struct {
-  bar string
 }
 
 type Bar struct {
-  foo string
 }