modules/constant: Add ibmod_constant which adds a configuration-time constant feature.  RNS-1001
modules/constant.h: Fix issue with Constant::get. RNS-1001
predicate/ibmod_predicate.hpp: Fix issues with external predicate API.
predicate: Add ibmod_predicate_constant which provides the constant function.
predicate: Add files missing from previous commit.
predicate/ibmod_predciate_constant.cpp: Include order.
various: Whitespace cleanup.
modules/constant.h: Add final newline.
modules/constant.h: Add missing declaration.
predicate/ibmod_predicate_constant.cpp: Spell check.
predicate/ibmod_predicate.cpp: Add documentation on set_predicate_vars.
predicate/ibmod_predicate.cpp: Add documentation on PredicateTrace.
predicate/lua_frontend: Spell check.
predicate/reference: Spell check.
codecheck: Update dictionary.
perdicate: Rewrite introduction; now introduction.txt instead of predicate.md.
CHANGES.md: Add note about predicate documentation.
predicate/ibmod_predicate.cpp: Fix uninitialized members issue. CID 1062882
predicate: Put table of contents of left for all documentation.
clipp/pb_consumer: Add support for writing to arbitrary ostream.  Also appends if file exists.