Fix many typos in docs and comments

This commit 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.