Rename the header <janet/janet.h> to <janet.h>

Makes it easier to use and remember, and makes the
variable `module/*headerpath*` make more sense.

examples/numarray/numarray.c

@@ -1,5 +1,5 @@
 #include <stdlib.h>
-#include <janet/janet.h>
+#include <janet.h>
 
 typedef struct {
     double * data;