core: Add blob + data stream abstraction

A blob is a piece of data of variable length that belongs in a blob list
while a data stream is a buffer used to move data between blobs among
other things.

Signed-off-by: Chloe M. <chloe@mirocom.org>

core/blob_subr.c

@@ -0,0 +1,25 @@
+/*
+ * Copyright (c) 2026, Chloe M.
+ * Provided under the BSD-3 clause
+ */
+
+#include <string.h>
+#include "blobchain/blob.h"
+#include "blobchain/memlib.h"
+
+struct blob *
+blob_allocate(struct data_stream *stmp)
+{
+    struct blob *bp;
+
+    bp = xmalloc(sizeof(*bp));
+    bp->length = (stmp != NULL) ? stmp->length : 0;
+    if (stmp == NULL) {
+        bp->data = NULL;
+    } else {
+        bp->data = xmalloc(stmp->length);
+        memcpy(bp->data, stmp->data, stmp->length);
+    }
+
+    return bp;
+}