Emitter: finalize no longer tries to load builtin functions.

Functions for a class will always be readonly. The var chain loader...skips
everything readonly. So this was just a no-op.
symtab: Drop symtab->lily_main.

Also, remove the comment about it not being a register, because it is in a
register and it has been for a while.
The symtab now holds classes in a linked list. It begins.