Reorganised source files to make navigation easier

All tests still pass (slightly to my surprise)

src/arith/real.c

@@ -0,0 +1,26 @@
+/*
+ * real.c
+ *
+ * functions for real number cells.
+ *
+ * (c) 2017 Simon Brooke <simon@journeyman.cc>
+ * Licensed under GPL version 2.0, or, at your option, any later version.
+ */
+
+#include "conspage.h"
+#include "consspaceobject.h"
+#include "read.h"
+
+/**
+ * Allocate a real number cell representing this value and return a cons 
+ * pointer to it.
+ * @param value the value to wrap;
+ * @return a real number cell wrapping this value.
+ */
+struct cons_pointer make_real( long double value ) {
+    struct cons_pointer result = allocate_cell( REALTAG );
+    struct cons_space_object *cell = &pointer2cell( result );
+    cell->payload.real.value = value;
+
+    return result;
+}