#24 White space handling

[Joachim Schrod]

tex.xdy and keep-blanks.xdy should work together, but
they don't.
tex.xdy establishs white space compression, but
keep-blanks.xdy cannot stop it.

Either keep-blanks must be able to delete merge rules
again.

Or tex.xdy must be able to activate its merge rules
later, and keep-blanks can prevent that activation.

In any case, the order of module loading is relevant:
It's only sensible to load keep-blanks if tex.xdy is
already loaded, this is not checked yet.