Trim blank lines around verbatim text

[aantron]

Odoc currently produces output for code blocks that looks like this:

Note the blank line at the top of the code block.

By comparison, ocamldoc produces the expected:

It is common to have the starting delimiter of a raw code or verbatim text block on a separate line; likewise the ending delimiter:

{[
val parse_html : char stream   -> signal stream
val write_html : signal stream -> char stream
val parse_xml  : char stream   -> signal stream
val write_xml  : signal stream -> char stream
]}

The two extra newlines should not affect the interpretation or presentation of the code block. To get this, the commit in this PR passes the accumulated text through the existing remove_opening_blanks and remove_closing_blanks.

The result is

In addition to code blocks, this bug is also present for verbatim text blocks, but it is more difficult to observe because leading line breaks are ignored in pre content when parsing HTML (reference, see 'A start tag whose tag name is one of: "pre", "listing"'). For code blocks, the leading token generated by odoc in pre elements ends up being a nested <code> tag, not a line break, resulting in the visual bug.

Even though it's not visible, I fixed the bug in verbatim blocks as well, so that any future introduction of nested markup for verbatim blocks does not cause visual artifacts.

[dbuenzli]

This should fix ocaml/odoc#39

[lpw25]

This looks correct to me. I was going to say that it should only remove blanks up to and including the first newline, but it seems that is exactly what remove_opening_blanks does (modulo a bug I just spotted in it which I will fix shortly).

[aantron]

I suggest also changing the function names to be more self-descriptive. I also didn't realize that remove_opening_blanks and its sibling remove one line each. Fortunately, I read them before starting to write my own :)