Reorganised source files to make navigation easier

All tests still pass (slightly to my surprise)

src/arith/peano.h

@@ -0,0 +1,58 @@
+/**
+ * peano.h
+ *
+ * Basic peano arithmetic
+ *
+ * (c) 2017 Simon Brooke <simon@journeyman.cc>
+ * Licensed under GPL version 2.0, or, at your option, any later version.
+ */
+
+#include "consspaceobject.h"
+
+#ifndef PEANO_H
+#define PEANO_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * Add an indefinite number of numbers together
+ * @param env the evaluation environment - ignored;
+ * @param frame the stack frame.
+ * @return a pointer to an integer or real.
+ */
+    struct cons_pointer
+     lisp_add( struct stack_frame *frame, struct cons_pointer env );
+
+/**
+ * Multiply an indefinite number of numbers together
+ * @param env the evaluation environment - ignored;
+ * @param frame the stack frame.
+ * @return a pointer to an integer or real.
+ */
+    struct cons_pointer
+     lisp_multiply( struct stack_frame *frame, struct cons_pointer env );
+
+/**
+ * Subtract one number from another.
+ * @param env the evaluation environment - ignored;
+ * @param frame the stack frame.
+ * @return a pointer to an integer or real.
+ */
+    struct cons_pointer
+     lisp_subtract( struct stack_frame *frame, struct cons_pointer env );
+
+/**
+ * Divide one number by another.
+ * @param env the evaluation environment - ignored;
+ * @param frame the stack frame.
+ * @return a pointer to an integer or real.
+ */
+    struct cons_pointer
+     lisp_divide( struct stack_frame *frame, struct cons_pointer env );
+
+#ifdef __cplusplus
+}
+#endif
+#endif                          /* PEANO_H */