Code Blocks must always be AtomicStrings

Fixes #340. The "inline" TreeProcessor runs before the "prettify"
TreeProcessor, but the "smarty" TreeProcessor (wich is just another
instance of `InlineProcessor`) runs after the "prettify" TreeProcessor.
The problem was that the "prettify" TreeProcessor was losing the
AtomicString quality of the text of code blocks (any operation
on a string creates a new string. When that string is an AtomicString,
the new string must explicitly be declared as an AtomicString.
As the "prettify" TreeProcessor cleans up newlines on code blocks,
it was changing the AtomicString to a normal string. And as
`InlineProcessor` identifies what elements to skip solely by whether
the text is an AtomicString, the "smarty" instance was running on
code blocks.

Importantly, I added a test of code blocks and spans for smarty,
so this shouldn't break again.