Found with gcc's -Wmissing-declarations which gives warnings like
../src/tinyexpr.cpp:61:5: warning: no previous declaration for ‘int get_arity(int)’ [-Wmissing-declarations]
61 | int get_arity(const int type) {
The same warnings show up for builtin functions like builtin_bg because they
currently don't include their own headers. I left that.
Also reformat the touched files.
This new file is supposed to encapsulate all of the logic around
reacting to variable changes, as opposed to the environment core.
This is to help break up the env.cpp monolith.
