Refactor NIP19 implementation and add tests

Closes: #837

damus-c/block.h

@@ -0,0 +1,56 @@
+//
+//  block.h
+//  damus
+//
+//  Created by William Casarin on 2023-04-09.
+//
+
+#ifndef block_h
+#define block_h
+
+#include "nostr_bech32.h"
+#include "str_block.h"
+
+#define MAX_BLOCKS 1024
+
+enum block_type {
+    BLOCK_HASHTAG = 1,
+    BLOCK_TEXT = 2,
+    BLOCK_MENTION_INDEX = 3,
+    BLOCK_MENTION_BECH32 = 4,
+    BLOCK_URL = 5,
+    BLOCK_INVOICE = 6,
+};
+
+
+typedef struct invoice_block {
+    struct str_block invstr;
+    union {
+        struct bolt11 *bolt11;
+    };
+} invoice_block_t;
+
+typedef struct mention_bech32_block {
+    struct str_block str;
+    struct nostr_bech32 bech32;
+} mention_bech32_block_t;
+
+typedef struct block {
+    enum block_type type;
+    union {
+        struct str_block str;
+        struct invoice_block invoice;
+        struct mention_bech32_block mention_bech32;
+        int mention_index;
+    } block;
+} block_t;
+
+typedef struct blocks {
+    int num_blocks;
+    struct block *blocks;
+} blocks_t;
+
+void blocks_init(struct blocks *blocks);
+void blocks_free(struct blocks *blocks);
+
+#endif /* block_h */