Don't activate rand dependency when only the serde feature is giv…

…en. (#133)

Cargo.toml

@@ -30,7 +30,7 @@ serde_test = "1.0"
 [features]
 default  = ["std"]
 std      = ["num-traits/std"]
-serde    = ["dep:serde", "rand/serde1"]
+serde    = ["dep:serde", "rand?/serde1"]
 randtest = ["rand/std", "rand/std_rng"]
 rkyv     = ["rkyv_32"]
 rkyv_16  = ["dep:rkyv", "rkyv?/size_16"]