bug#20645: sxml xpath exports "filter"

[Matt Wette]

sxml xpath exports "filter" which is incompatible with the common procedure of 
the same name.

I suggest renaming unless it breaks a lot of code.

I use

  #:use-module ((sxml xpath)
                #:renamer (lambda (s) (if (eq? s 'filter) 'xp-filter s)))

but would rather other agree that renaming in sxml/xpath.scm is better.

Matt