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/fiber.h

@@ -23,7 +23,9 @@
 #ifndef JANET_FIBER_H_defined
 #define JANET_FIBER_H_defined
 
+#ifndef JANET_AMALG
 #include <janet/janet.h>
+#endif
 
 extern JANET_THREAD_LOCAL JanetFiber *janet_vm_fiber;