fuzz: add size limit to regex building

The fuzzer sometimes runs into situations where it builds regexes that
can take a while to execute, such as `\B{10000}`. They fit within the
default size limit, but the search times aren't great. But it's not a
bug. So try to decrease the size limit a bit to try and prevent
timeouts.

We might consider trying to optimize cases like `\B{10000}`. A naive
optimization would be to remove any redundant conditional epsilon
transitions within a single epsilon closure, but that can be tricky to
do a priori. The case of `\B{100000}` is probably easy to detect, but
they can be arbitrarily complex.

Another way to attack this would be to modify, say, the PikeVM to only
compute whether a conditional epsilon transition should be followed once
per haystack position. Right now, I think it is re-computing them even
though it doesn't have to.

Author: BurntSushi

fuzz/fuzz_targets/fuzz_regex_lite_match.rs

@@ -55,6 +55,7 @@ fuzz_target!(|case: FuzzCase| {
         .dot_matches_new_line(case.dot_matches_new_line)
         .swap_greed(case.swap_greed)
         .ignore_whitespace(case.ignore_whitespace)
+        .size_limit(1<<20)
         .build() else { return };
     re.is_match(case.haystack);
 });

fuzz/fuzz_targets/fuzz_regex_match.rs

@@ -59,6 +59,7 @@ fuzz_target!(|case: FuzzCase| {
         .ignore_whitespace(case.ignore_whitespace)
         .unicode(case.unicode)
         .octal(case.octal)
+        .size_limit(1<<20)
         .build() else { return };
     re.is_match(case.haystack);
 });