Move everything out of functions.h/variables.h

[bates64]

functions.h and variables.h have always been a temporary measure to make it quicker to match stuff. They define a ton of random functions and variables that should really be declared elsewhere.

The ~~not-yet-enabled~~ warnings -Wimplicit -Wredundant-decls described in #366 are relevant here too, as they will enforce good use of header files.

Additionally, we should move all declarations out of .c files and into .h files.

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Steps to migrate functions from functions.h:

1. Pick a C file with nonstatic functions you'd like to make a header for
2. Create a header file (.h) next to the C file with an include guard
3. Add an extern declaration for global variables defined in the C file (usually marked with g at the start of the variable name)
4. Add a declaration for every non-static function in the C file
5. Remove declarations from functions.h and variables.h
6. Add a #include "myfoo.h" to every file that uses variables or functions declared in your header

Include guard example

#ifndef _MYFOO_H_
#define _MYFOO_H_

// ...

#endif

[bates64]

If we reenable clang-tidy we should be able to automatically shout about declarations that aren't in headers