Added the beginnings of hashmap but does not yet compile.

src/memory/hashmap.c

@@ -0,0 +1,61 @@
+/*
+ * hashmap.c
+ *
+ * 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.
+ */
+
+#include "arith/integer.h"
+#include "memory/consspaceobject.h"
+#include "memory/hashmap.h"
+
+/**
+ * Get the hash value for the cell indicated by this `ptr`; currently only
+ * implemented for string like things.
+ */
+uint32_t get_hash(struct cons_pointer ptr)
+{
+    struct cons_space_object *cell = &pointer2cell(ptr);
+    uint32_t result = 0;
+
+    switch (cell->tag.value)
+    {
+    case KEYTV:
+    case STRINGTV:
+    case SYMBOLTV:
+        result = cell->payload.string.hash;
+    default:
+        // TODO: Not Yet Implemented
+        result = 0;
+    }
+
+    return result;
+}
+
+/**
+ * A lisp function signature conforming wrapper around get_hash, q.v..
+ */
+struct cons_pointer lisp_get_hash(struct stack_frame *frame,
+                                  struct cons_pointer frame_pointer,
+                                  struct cons_pointer env)
+{
+    return make_integer(get_hash(frame->arg[0]), NIL);
+}
+
+/**
+ * Make a hashmap with this number of buckets.
+ */
+struct cons_pointer make_hashmap( uint32_t n_buckets) {
+    struct cons_pointer result = make_vso(HASHTAG,
+    (sizeof(struct cons_pointer) * (n_buckets + 1)) +
+    (sizeof(uint32_t) * 2));
+
+    //  TODO: fill in the payload!
+
+    struct hashmap_payload *payload  = 
+        (struct hashmap_payload *) &pointer_to_vso(result)->payload;
+
+    return result;
+}