This patches takes the options work to it's logical conclusion: A single
flat dictionary of OptionKey: UserOptions. This allows us to simplify a
large number of cases, as we don't need to check if an option is in this
dict or that one (or any of 5 or 6, actually).
D lang compilers have an option -release (or similar) which turns off
asserts, contracts, and other runtime type checking. This patch wires
that up to the b_ndebug flag.
Fixes#7082