This is in preparation for a subsequent commit where we add special
handling for paths in the template, allowing them to be qualified
differently in different contexts.
Previously we had:
- Multiple rules
- Each rule had its pattern parsed as an expression, path etc
This meant that there were two levels at which there could be multiple
rules.
Now we just have multiple rules. If a pattern can parse as more than one
kind of thing, then they get stored as multiple separate rules.
We also now don't have separate fields for the different kinds of things
that a pattern can parse as. This makes adding new kinds of things
simpler.
Previously, add_search_pattern would construct a rule with a dummy
replacement. Now the replacement is an Option. This is slightly cleaner
and also opens the way for parsing the replacement template as the same
kind of thing as the search pattern.
