Introduce multi-purpose data/len members in struct cmark_node. This
is mainly used to store literal text for inlines, code and HTML blocks.

Move the content strbuf for blocks from cmark_node to cmark_parser.
When finalizing nodes that allow inlines (paragraphs and headings),
detach the strbuf and store the block content in the node's data/len
members. Free the block content after processing inlines.

Reduces size of struct cmark_node by 8 bytes.

Closes #290.

The `alloc` member wasn't initialized.

This also allows to add an assertion in `chunk_rtrim` which doesn't
work for alloced chunks.

It's possible that cmark_chunk_set_cstr is called with a substring
(suffix) of the current string. Delay freeing of the chunk content
to handle this case correctly.

Fixes issue #139.