other. Ensure headers in include/ can be included in any order
(except for econfig-spec.h, which is special). New header types.h to
help avoid inclusion cycles. Sort include directives. Remove some
superflous includes.
(nati, populace): Use it.
(populace, nati): Change hap, pct from float to double, because they
are used as double.
(nati): Print happiness needed with just two decimals, like the other
levels.
