Continuous fuzzing with Fuzzit (#1042)

Enables automated fuzzing on Fuzzit. Runs fuzz regression tests
every push and PR. Runs full fuzzing every push. Fuzzit emails
if it finds crashes.

Uses the existing fuzz targets:
* translate-module - Fuzz valid WebAssembly modules.
* reader-parse - Fuzz IR text format parsing.

cranelift/fuzz/fuzz_reader_parse_test.rs

@@ -6,6 +6,7 @@ use std::str;
 
 fuzz_target!(|data: &[u8]| {
     if let Ok(s) = str::from_utf8(data) {
-        let _ = cranelift_reader::parse_test(s, None, None);
+        let options = cranelift_reader::ParseOptions::default();
+        let _ = cranelift_reader::parse_test(s, options);
     }
 });