- Grow the underlying buffer of SanitizeHTML's strings.Builder to 3/4 of the
raw HTML from the start, to reduce the amount of iterative allocations. This
number is a complete guesstimation, but it sounds reasonable to me.
- Add a `absoluteURLParsedBase` function to avoid parsing baseURL over and over.
Calls to urllib.AbsoluteURL take a bit less than 10% of the time spent in
parser.ParseFeed, completely parsing an url only to check if it's absolute, and
if not, to make it so.
Checking if it starts with `https://` or `http://` is usually enough to find if
an url is absolute, and if is doesn't, it's always possible to fall back to
urllib.AbsoluteURL.
This also comes with the advantage of reducing heap allocations, as most of the
time spent in urllib.AbsoluteURL is heap-related (de)allocations.
