Add separate GDB pretty-printer for empty structs
Add separate GDB pretty-printer for empty structs

Use a class without children() method for printing empty structs.
Presence of this method makes GDB's variable objects interface act like
if the struct had children.

GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust