Whitespace cleanup

[lassik]

From the MELPA PR thread:

format-all-fix-trailing-whitespace is just an ad-hoc reimplementation of whitespace-cleanup. I suggest you remove it in any case, since if users care about this, they can add whitespace-cleanup to their before-save hook.

The thing is that many (probably most) of the external formatters already do their own whitespace cleanup as part of their normal operation. It usually can't even be turned off. I think they are doing the right thing, since accidental whitespace changes are a common annoyance in version control diffs. I can't imagine anyone wanting to auto-format their code to a precise standard, yet not remove trailing whitespace.

format-all-fix-trailing-whitespace exists because a few formatters don't fix whitespace for some reason. The point of the format-all package is to provide a consistent interface to formatters, so that users can forget about their differences (I want to give people "the luxury of ignorance"). If most formatters clean up whitespace but some of them randomly don't, I think that violates this expectation and we should patch it up. The ideal would be to convince the people making those external formatters to add whitespace cleanup, but there are so many of them that it would take years.

I think Emacs' own whitespace-cleanup does too much for our purposes. It relies on indent-tabs-mode and tab-width to fix tabs vs spaces. But the Emacs values of those variables may not agree with the formatter's idea of tabs vs spaces and it would be very complex to match them up reliably for all formatters (and in my opinion fruitless to expect users to always have them matched up). Indeed, I have personally fallen into a pattern where I often don't even bother to configure Emacs' indentation and whitespace rules anymore - I just keep doing format-all constantly and the formatter makes everything right with no configuration.

To clarify, I think most if not all external formatters fix tabs vs spaces (to their idea of what it should be) but some of them don't fix trailing whitespace. We can easily lop off trailing whitespace in with an Emacs function without caring about tabs vs spaces. Hence the custom function.