Add integrity checking to tests and fix various bugs

[alangpierce]

Fixes #115

This commit adds a new IntegrityCheckingMagicString class that subclasses
MagicString and does an integrity check operation, which is used in test code.

This revealed 5 bugs, all of which are also fixed in this commit:

• Within indent, the code was calling chunk.split, then making an attempt at
  properly updating byStart and byEnd. But it wasn't properly updating
  lastChunk. Rather than calling chunk.split, we can just call
  this._splitChunk, which updates byStart, byEnd, and lastChunk
  correctly.
• move could sometimes assign undefined as a next pointer, which is
  inconsistent since all other cases use null. Not a really big deal, but nice
  to be consistent.
• overwrite was attempting to do a linked list removal in a way that wasn't
  properly updating all state ( overwrite method can cause state corruption #115 ). I tried changing the code to remove the
  nodes properly, but the details seemed to get really subtle, especially
  accounting for moved chunks, and it didn't seem like the complexity was worth
  it, so I effectively just reverted optimisation - remove empty chunks when overwriting #113 to fix the issue. But now with the
  new integrity checking, it should be a lot easier to get the details right if
  that's desirable.
• trimStart and trimEnd were attempting to update byStart and byEnd, but
  they didn't properly set byEnd of the newly-created chunk created by
  chunk.trimStart/chunk.trimEnd. Setting the byEnd for that new chunk
  seems to fix things.
• trimEnd was setting this.lastChunk = chunk.next, which makes sense for the
  first iteration (when working with the very last chunk in the linked list),
  but doesn't make sense for later iterations, since it meant that some chunks
  just get dropped. We now only run that line on the last chunk.

[Rich-Harris]

This is awesome, thanks so much! Released this as 0.22.

How would you feel about having admin access to this repo? I think it's reasonably stable at this point, but increasing bus factor is never a bad idea. No pressure though, totally your call 😀

[alangpierce]

Sure, that would be great. 😃

[Rich-Harris]

done! welcome aboard 🎉 Added you as an owner of the npm package as well, in case you need to cut a release and I'm on vacation/whatever