Otherwise we can get quadratic increase in size with deeply
nested structures.

See #355.

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.

Reduces size of struct cmark_node by 8 bytes.

Use zero-terminated C strings instead of cmark_chunks without storing
the length. This introduces a few additional strlen computations,
but overhead should be low.

Allows to reduce size of struct cmark_node later.

Use zero-terminated C strings instead of cmark_chunks without storing
the length. The length of code literals will be readded in a later
commit. strlen overhead for code info should be negligible.

Reduces size of struct cmark_node by 8 bytes.

With current HTML escaping, sometimes we may produce an XML tag like

    <code_block>  </code_block>

Many XML parsers consider these spaces insignificant and strip them
out but we need this. There's actually a test case like this in
spec.txt (search "A code block can have all empty lines as its
content:") and without proper hinting, an external xml->html converter
will fail the spec.

XML standard covers this case. If xml:space is "preserve", then
whitespaces are significant and should be kept. Add this hint for
text, code, code_block, html_inline and html_block tags.

They're not supported by MSVC.

Newer MSVC versions support enough of C99 to be able to compile cmark
in plain C mode. Only the "inline" keyword is still unsupported.
We have to use "__inline" instead.