Reorganised source files to make navigation easier

All tests still pass (slightly to my surprise)

src/arith/integer.c

@@ -0,0 +1,47 @@
+/*
+ * integer.c
+ *
+ * functions for integer cells.
+ *
+ * (c) 2017 Simon Brooke <simon@journeyman.cc>
+ * Licensed under GPL version 2.0, or, at your option, any later version.
+ */
+
+#define _GNU_SOURCE
+#include <math.h>
+#include <stdio.h>
+
+#include "conspage.h"
+#include "consspaceobject.h"
+#include "read.h"
+
+/**
+ * return the numeric value of this cell, as a C primitive double, not
+ * as a cons-space object. Cell may in principle be any kind of number,
+ * but only integers and reals are so far implemented.
+ */
+long double numeric_value( struct cons_pointer pointer ) {
+    long double result = NAN;
+    struct cons_space_object *cell = &pointer2cell( pointer );
+
+    if ( integerp( pointer ) ) {
+        result = cell->payload.integer.value * 1.0;
+    } else if ( realp( pointer ) ) {
+        result = cell->payload.real.value;
+    }
+
+    return result;
+}
+
+/**
+ * Allocate an integer cell representing this value and return a cons pointer to it.
+ */
+struct cons_pointer make_integer( long int value ) {
+    struct cons_pointer result = allocate_cell( INTEGERTAG );
+    struct cons_space_object *cell = &pointer2cell( result );
+    cell->payload.integer.value = value;
+
+    dump_object( stderr, result );
+
+    return result;
+}