Added the beginnings of hashmap but does not yet compile.

src/memory/hashmap.h

@@ -0,0 +1,36 @@
+/*
+ * hashmap.h
+ *
+ * Basic implementation of a hashmap.
+ *
+ * (c) 2021 Simon Brooke <simon@journeyman.cc>
+ * Licensed under GPL version 2.0, or, at your option, any later version.
+ */
+
+#ifndef __psse_hashmap_h
+#define __psse_hashmap_h
+
+#include "memory/consspaceobject.h"
+#include "memory/vectorspace.h"
+
+/**
+ * The payload of a hashmap. The number of buckets is assigned at run-time,
+ * and is stored in n_buckets. Each bucket is something ASSOC can consume: 
+ * i.e. either an assoc list or a further hashmap.
+ */
+struct hashmap_payload {
+    struct cons_pointer hash_fn;
+    uint32_t n_buckets;
+    uint32_t unused; /* for word alignment and possible later expansion */
+    struct cons_pointer buckets[];
+};
+
+uint32_t get_hash(struct cons_pointer ptr);
+
+struct cons_pointer lisp_get_hash(struct stack_frame *frame,
+                                  struct cons_pointer frame_pointer,
+                                  struct cons_pointer env);
+
+struct cons_pointer make_hashmap( uint32_t n_buckets, struct cons_pointer hash_fn);
+
+#endif