Prefer Fixnums to Numerics

Numerics are effectively integers and floats in Ruby, whereas Fixnums
are effectively just integers. Let's use slightly tighter bounds on our
types to catch any potential weird type errors sooner.

ext/cruzzy/cruzzy.c

@@ -78,7 +78,7 @@ static int proc_caller(const uint8_t *data, size_t size)
         );
     }
 
-    return NUM2INT(result);
+    return FIX2INT(result);
 }
 
 static VALUE c_fuzz(VALUE self, VALUE test_one_input, VALUE args)
@@ -120,7 +120,7 @@ static VALUE c_fuzz(VALUE self, VALUE test_one_input, VALUE args)
     // https://llvm.org/docs/LibFuzzer.html#using-libfuzzer-as-a-library
     int result = LLVMFuzzerRunDriver(&args_len, &args_ptr, proc_caller);
 
-    return INT2NUM(result);
+    return INT2FIX(result);
 }
 
 void Init_cruzzy()