Fix many typos in docs and comments

This PR corrects many typos found in scaladocs, comments and
documentation. It should reduce a bit number of PRs which fix one
typo.

There are no changes in the 'real' code except one corrected name of
a JUnit test method and some error messages in exceptions. In the case
of typos in other method or field names etc., I just skipped them.

Obviously this commit doesn't fix all existing typos. I just generated
in IntelliJ the list of potential typos and looked through it quickly.
It's a public method of a public class - in general I was avoiding the changes which could (let's say) break the compatibility etc. I'm sure that then I would have to explain why I changed such things and why it's safe and so on, and so on.