Use a hash table for anchors
Applied hash table optimisation to RemoveAnchorByNode. This function now takes
the anchor name as a parameter, so it can look in the correct bin.

In the case of FreeAttrs, we have the name already (since we found a name or
id attribute). In the case of FixAnchors, the anchor name could come from
either the name or id attribute, so we call the function separately for each
case, passing the appropriate attribute value.
Merge branch 'master' of https://github.com/uxproductivity/tidy-html5