Add gnus-summary-ext-search-article-p
Refactor: gnus-summary-ext-iterate-articles-safely
Lots of changes

Altered items: macro-utils,
gnus-summary-ext-iterate-articles-safely-1,
gnus-summary-ext-iterate-articles-safely,
gnus-summary-ext-count-parts, gnus-summary-ext-limit-to-num-parts,
gnus-summary-ext-limit-to-size, gnus-summary-ext-search-article-p,
gnus-summary-ext-read-limit-expression,
gnus-summary-ext-limit-expression
Update documentation
Update documentation

Altered items: gnus-summary-ext-limit-expression
Add gnus-summary-ext-saved-filters option

Allow storage of saved filters which can be used by
gnus-summary-ext-limit-expression

Altered items: gnus-summary-ext, gnus-summary-ext-saved-filters,
gnus-summary-ext-limit-expression
Function: gnus-summary-ext-limit-filter

Altered items: gnus-summary-ext-limit-filter, gnus-summary-ext
Update documentation

Altered items: gnus-summary-ext-saved-filters