Add function versions of macro API bindings.

This should help address #81. Also hide janet_exit
and janet_assert, as they are really meant for internal usage.
I have not verified that this yet actually works with Rust's
bindgen.

src/core/strtod.c

@@ -45,6 +45,7 @@
 
 #ifndef JANET_AMALG
 #include <janet.h>
+#include "util.h"
 #endif
 
 /* Lookup table for getting values of characters when parsing numbers. Handles