Fill the globals with position information

[masak]

• Create a new Bel::Pair:: type which we can use for the cons cells in the Bel::Globals.
• Actually use those when we compile Bel::Globals.
• Modify the Perl interpreter so that it notices when it has the position information ready, and uses it in the error message.

It strikes me that something also needs to be done for the fastfuncs and the fast operatives; but I'll leave that as out-of-scope for now.