Properties of Block values are marshalled lazily, which generally
improves performance considerably. Script users may also notice the
following differences:
- Block element properties can no longer be accessed by numerical
indexing of the `.c` field. The `.c` property now serves as an alias
for `.content`, so some filter that used this undocumented method
for property access may continue to work, while others will need to
be updated and use proper property names.
- The marshalled Block elements now have a `show` method, and a
`__tostring` metamethod. Both return the Haskell string
representation of the element.
- Block values now have the Lua type `userdata` instead of `table`.
a493c7029c