Typo in source code: "executer"

[mpboom]

The source code seems to contain quite a bit of references to an executer, while we think/believe the word executor is meant here. Even though it may introduce merge conflicts or a bit of discomfort for contributors, it seems like a good decision to replace the term with the correct spelling in a commit. This reduces confusion and lessens opportunity for future mistakes.

See attached PR for a search+replace action on the current state of the codebase.