blurb: get rid of spaces in paths

[serhiy-storchaka]

I suggest to rename the "Core and Builtins" directory to "Core_and_Builtins" and make blurb automatically converting underscores to spaces and back. This would help to select paths in terminal with mouse.

[encukou]

C API/ is similarly unpleasant to work with.

[hugovk]

Please see PRs #499 and python/blurb_it#332.

[merwok]

The PR is ready for review.

If Serhiy and Petr both approve, do we need a discussion on Discuss? Just a notice?

[encukou]

A notice would be appropriate. And it'll turn into a discussion :)