Only enable hex/std, alloc when hex is

`hex/std` and `hex/alloc` should only be included if optional
dependency `hex` is enabled`.

Add `?` so it is only included if the optional feature `hex` is enabled.

hashes/Cargo.toml

@@ -15,8 +15,8 @@ exclude = ["tests", "contrib"]
 
 [features]
 default = ["std"]
-std = ["alloc", "hex/std"]
-alloc = ["hex/alloc"]
+std = ["alloc", "hex?/std"]
+alloc = ["hex?/alloc"]
 serde = ["dep:serde", "hex"]
 # Smaller (but slower) implementation of sha256, sha512 and ripemd160
 small-hash = []