fuzz: improve target perf (#864)

This attempts to improve issue types like timeout, slow_unit
and speed, as seen in the following oss-fuzz performance report:
https://oss-fuzz.com/performance-report/libFuzzer_go-toml_fuzz_toml/libfuzzer_asan_go-toml/2023-05-11

ossfuzz/fuzz.go

@@ -12,7 +12,7 @@ import (
 )
 
 func FuzzToml(data []byte) int {
-	if len(data) >= 10240 {
+	if len(data) >= 2048 {
 		return 0
 	}