Add block comment support about wsedit

[LadyBoonami]

It should be fairly obvious that the current rendering strategy of mapping a line render function over the visible area does not work for this. Unlike multiline strings, multiline comments are way too common to just ignore them, so a new rendering method is needed.

Folding over the entire file, while staying true to the philosophy of "No caching means no caching bugs", is utterly impractical, since it would slow down rendering larger files to a crawl. And dear Lord, we have enough performance problems already.

So we need some way of caching rendering information, possibly in a symbol table. We can calculate the differences between two buffer states by making use of the fact that changes always occur near the focus, never at the ends. Aligning the buffer lines via prefLen should yield the point where the two buffers start to match, so only that area would need to be re-parsed every time.

This, in turn, requires a quick way to check large strings for equality, possibly by transparently integrating hashes into Buffer.

All these are sweeping changes I do not want to implement a mere week from launch, so I'll have to delay this until after release. But sooner or later, this will have to happen, since I'd consider block comments a fairly substantial feature and simply too common and important to ignore.

[LadyBoonami]

Done (1bb07db).