allow panics inside PEG parser (#5301)

This commit changes the default behavior of the PEG parser
to not recover from panics.  Previously, a panic would be caught
and converted to an error then we would report the error as
a syntax error. There should be no panics in the grammar and
if there are, they should be displayed to users so bugs can
be filed and fixed.

compiler/parser/api.go

@@ -15,7 +15,7 @@ func ParseZed(filenames []string, src string) (ast.Seq, *SourceSet, error) {
 	if err != nil {
 		return nil, nil, err
 	}
-	p, err := Parse("", []byte(sset.Text))
+	p, err := Parse("", []byte(sset.Text), Recover(false))
 	if err != nil {
 		return nil, nil, convertErrList(err, sset)
 	}