Move public headers into a separate include/woff2 directory

This is a first step to make WOFF2 a shared library. Public headers are moved
in their own directory to make the public API clearer. This is similar to
the c/include/brotli/ directory in Brotli. "using" commands are also removed
from the public headers, so that callers won't have unexpected side effect
when including the files.

Makefile

@@ -1,6 +1,6 @@
 OS := $(shell uname)
 
-CPPFLAGS = -I./brotli/c/include/ -I./src
+CPPFLAGS = -I./brotli/c/include/ -I./src -I./include
 
 AR ?= ar
 CC ?= gcc