Previously we omitted extra whitespace for single PCDATA/CDATA children, but in
mixed content there was extra indentation before/after text nodes.

One of the problems with that is that the text that you saved is not exactly
the same as the parsing result using default flags (parse_trim_pcdata helps).

Another problem is that parse-format cycles do not have a fixed point for mixed
content - the result expands indefinitely. Some XML libraries, like Python
minidom, have the same issue, but this is definitely a problem.

Pretty-printing mixed content is hard. It seems that the only other sensible
choice is to switch mixed content nodes to raw formatting. In a way the code in
this change is a weaker version of that - it removes indentation around text
nodes but still keeps it around element siblings/children.

Thus we can switch to mixed-raw formatting at some point later, which will be
a superset of the current behavior.

To do this we have to either switch at the first text node (.NET XmlDocument
does that), or scan the children of each element for a possible text node and
switch before we output the first child.

The former behavior seems non-intuitive (and a bit broken); unfortunately, the
latter behavior can cost up to 20% of the output time for trees *without* mixed
content.

Fixes #13.