Rework `make tidy`

[brson]

Our tidy script takes all the files to look at as arguments and we keep hitting the maximum number of arguments.

[emberian]

@brson don't think this is an issue anymore?

[pnkfelix]

part of #8058 (or at least it might be, sounds like it might not be an issue any more; I'll have to check I guess)

[emillon]

In mk/tests.mk, tidy.py is invoked through xargs -n 10, effectively limiting the number of arguments.

However, as an experiment, I rewrote the find + grep + filter_out logic inside tidy.py using os.walk in order to limit the creation of child processes. It's 30% faster (5.1s instead of 7.2s) but I think that it's way less readable: see emillon/rust@f8344f0c (not ready yet).

[alexcrichton]

We seem to be managing ok with the system we have today. As @emillon pointed out, we're not hitting the maximum number of arguments any more because of xargs, so there doesn't appear to be any pressing matter any more.