device_gfx_interface and memory system improvements: [Alex Jackson]

Added macros to facilitate declaring gfxdecode info arrays as members
of a device class.

AM_SHAREs in a device's internal address map or its default address map are
now tagmapped as children of that device rather than siblings (analogous
to how handlers in internal/default address maps are scoped).

Converted the Namco C45 to device_gfx_interface.