Now that ASTs have parse positions and the compiler will return the
position of the last error, use that in falco rules to return errors
within condition strings instead of reporting the position as the
beginning of the condition.
This led to a change in the filter_ruleset interface--now, an ast is
compiled to a filter before being passed to the filter_ruleset
object. That avoids polluting the interface with a lot of details
about rule_loader contexts, errors, etc. The ast is still provided in
case the filter_ruleset wants to do indexing/analysis of the filter.
Signed-off-by: Mark Stemm <mark.stemm@gmail.com>