Add source amalgamation

The amalgamated source concatenates all sources
to a file janet.c which can be used for
embedding janet, much in the same way as sqlite
or mongoose.

src/core/capi.c

@@ -20,9 +20,11 @@
 * IN THE SOFTWARE.
 */
 
+#ifndef JANET_AMALG
 #include <janet/janet.h>
 #include "state.h"
 #include "fiber.h"
+#endif
 
 void janet_panicv(Janet message) {
     if (janet_vm_fiber != NULL) {