replace ec3_reader and ec3_writer with a unified image and tag io interface

src/string-table.h

@@ -3,10 +3,19 @@
 
 #include <blue/core/btree.h>
 
+struct string_table_entry {
+	b_btree_node e_hash_node;
+	b_btree_node e_offset_node;
+	size_t e_hash;
+	size_t e_offset;
+	char e_str[];
+};
+
 struct string_table {
 	b_btree s_hash_tree;
 	b_btree s_offset_tree;
 	size_t s_next_offset;
+	size_t s_nr_entries;
 };
 
 extern void string_table_init(struct string_table *out);