G2P type in a dedicated package

For future global variable access via `Blackbox`, all type generated by
Go2Pins are now stored in a dedicated package `structs`.
All convenient modifications has been done to let Go2Pins in a valid
state.

 * boilerplate/Makefile, boilerplate/main.go,
   boilerplate/structs/structs.go,
   decl/decl.go,
   main.go,
   tools/blackbox.go,
   transform/meta.go : Here.
1 job for hm/global in 2 minutes and 11 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #129483
debian-stable-gcc

00:02:11