Move the Block helper type to its own file

Collapse the various standalone functions for parsing block data, and
refactor consumers to initialize a Block with given data and access its
members as needed.

Closes: https://github.com/damus-io/damus/pull/1496
Signed-off-by: William Casarin <jb55@jb55.com>

damus/ContentParsing.swift

@@ -27,7 +27,7 @@ func parsed_blocks_finish(bs: inout note_blocks, tags: TagsSequence?) -> Blocks
     while (i < bs.num_blocks) {
         let block = bs.blocks[i]
 
-        if let converted = convert_block(block, tags: tags) {
+        if let converted = Block(block, tags: tags) {
             out.append(converted)
         }