This commit adds support for iterating over note tags and tag elements.
This is zero-copy and heavily leverages rust lifetimes to ensure we
always have access within a transaction and note reference.

New types
=========

NdbStr - ndb note strings, offsets into the note string table
NdbStrVariant - ndb note string variants. can be 32-byte values or strings
Tag - A tag ["e", "abcdef..."]
Tags - Note tags [["hi", "3"], ["proxy", "..."]]
TagIter - An iterator over tag elements, producing NdbStr's
TagsIter - An iterator over tags elements, producing Tag's

Usage
=====

for tag in note.tags().iter() {
    for nstr in tag.iter() {
        match nstr.variant() {
	    NdbStrVariant::Str(s) => // string
	    NdbStrVariant::Id(s) =>  // 32-byte id
	}
    }
}

Changelog-Added: Add tags and tag iterators to Note
Fixes: #2
Cc: [email protected]

Signed-off-by: William Casarin <[email protected]>