message: rename NewMsg to New

Should have done this when relocating this from container. Now is a good time to rename it before v0.3.x.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/outcome/shim.go

@@ -78,7 +78,7 @@ const shimName = "shim"
 // Shim does not return.
 func Shim(msg message.Msg) {
 	if msg == nil {
-		msg = message.NewMsg(log.Default())
+		msg = message.New(log.Default())
 	}
 	shimEntrypoint(direct{msg})
 }