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.
(sect_ca, ship_ca, land_ca, plague_stages, empfile):
Add plague stages symbol table.
(treaty_ca, loan_ca, agreement_statuses, empfile):
Add agreement status symbol table.
Combine treatystatuses (TS_FREE, TS_SIGNED and TS_PENDING)
and loan statuses (LS_FREE, LS_SIGNED and LS_PENDING)
into a single agreement status.
(pchr_ca, level, empfile): Add packing symbol table.
