Add fuzz tests, rename fuzz tests and fix panic found by fuzzing (#72)

* begin adding better fuzz test

* finish adding working fuzzer

* implement mutateipv4

* fuzzer finds a panic in ICMP client on receive empty payload

* rename fuzz tests to reflect fuzz methodology

internal/prand.go

@@ -19,7 +19,7 @@ func Prand32[T ~uint32](seed T) T {
 }
 
 // Prand32 generates a pseudo random number from a seed.
-func Prand64[T ~uint64](seed T) T {
+func Prand64[T ~uint64 | ~int64](seed T) T {
 	seed ^= seed << 13
 	seed ^= seed >> 7
 	seed ^= seed << 17