Header consolidation - main.h, misc.h, and share.h merge to advent.h.

Header consolidation - main.h, misc.h, and share.h merge to advent.h.

For a project this small size with a small type ontology, it's better
C style to have one header where all the assumptions about types and
structures are immediately visible.

This is a first step towards turning all that global state into a handful
of structures - probably just two, one saveable as a binary game state.