Add API for changesets

[noteflakes]

Using the bundled SQLite session extension, changesets can be generated that track changes to data, which can then be represented in a blob. Here's how the Extralite API might look:

changeset = db.track_changes(:foo, :bar) do |cs|
  cs.attach(:baz) # attach more tables

  db << "insert into foo values (1, 2, 3)"
end

# iterate through changes
changeset.each { |c| p c }

# get all changes
changeset.to_a

# get changeset blob
changeset.to_blob

# get patchset
changeset.to_blob_patchset

# load from blob
changeset = Extralite::Changeset.new(blob)

# invert
changeset.invert(db)

# apply to DB
changeset.apply(db)