move util.c to util.h

This allows single-source compilation and whole-program optimization using something like:

make CFLAGS='-flto -ffunction-sections -fdata-sections -Wl,--gc-sections'

Furthermore, move more compositor-agnostic utility functions/macros to util.h,
similar to the original dwm sources.

README.md

@@ -10,7 +10,7 @@ primarily in terms of functionality, and secondarily in terms of
 philosophy. Like [dwm], dwl is:
 
 - Easy to understand, hack on, and extend with patches
-- One C source file (or a very small number) configurable via `config.h`
+- One C source file configurable via `config.h`
 - Tied to as few external dependencies as possible
 
 ## Getting Started: