Reorganised source files to make navigation easier

All tests still pass (slightly to my surprise)

src/arith/real.h

@@ -0,0 +1,32 @@
+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+
+/*
+ * File:   real.h
+ * Author: simon
+ *
+ * Created on 14 August 2017, 17:25
+ */
+
+#ifndef REAL_H
+#define REAL_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * 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 );
+
+#ifdef __cplusplus
+}
+#endif
+#endif                          /* REAL_H */