sdrawkcab fubotorp lahsraM (gogo#560)

* Marshal protobuf backwards

Reverse all the logic so that objects are serialized backward (when
possible), to avoid recursively (and repeatedely) pre-computing Size().

* Reverse EncodeInternalExtensions

* Use MarshalToSizedBuffer in XXX_Marshall

Because the length of the buffer is already known.

* Add new protobuf message to test recursive maps

* Increase likelihood that generated maps are deep

* Run make all

go.mod

@@ -1,3 +1,6 @@
 module github.com/gogo/protobuf
 
-require github.com/kisielk/errcheck v1.1.0 // indirect
+require (
+	github.com/kisielk/errcheck v1.2.0 // indirect
+	github.com/kisielk/gotool v1.0.0 // indirect
+)

go.sum

@@ -1,6 +1,10 @@
 github.com/kisielk/errcheck v1.1.0 h1:ZqfnKyx9KGpRcW04j5nnPDgRgoXUeLh2YFBeFzphcA0=
 github.com/kisielk/errcheck v1.1.0/go.mod h1:EZBBE59ingxPouuu3KfxchcWSUPOHkagtvWXihfKN4Q=
+github.com/kisielk/errcheck v1.2.0 h1:reN85Pxc5larApoH1keMBiu2GWtPqXQ1nc9gx+jOU+E=
+github.com/kisielk/errcheck v1.2.0/go.mod h1:/BMXB+zMLi60iA8Vv6Ksmxu/1UDYcXs4uQLJ+jE2L00=
 github.com/kisielk/gotool v1.0.0 h1:AV2c/EiW3KqPNT9ZKl07ehoAGi4C5/01Cfbblndcapg=
 github.com/kisielk/gotool v1.0.0/go.mod h1:XhKaO+MFFWcvkIS/tQcRk01m1F5IRFswLeQ+oQHNcck=
 golang.org/x/tools v0.0.0-20180221164845-07fd8470d635 h1:2eB4G6bDQDeP69ZXbOKC00S2Kf6TIiRS+DzfKsKeQU0=
 golang.org/x/tools v0.0.0-20180221164845-07fd8470d635/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
+golang.org/x/tools v0.0.0-20181030221726-6c7e314b6563 h1:NIou6eNFigscvKJmsbyez16S2cIS6idossORlFtSt2E=
+golang.org/x/tools v0.0.0-20181030221726-6c7e314b6563/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=