Add hashes to io dev-depencies and require hex

The tests in `hash` require hex.  Previously this was enabled when
`alloc` was enabled.  Now `alloc` doesn't enable `hex` add `hashes` to
`dev-dependecies` and set it to require `hex`

io/Cargo.toml

@@ -23,6 +23,9 @@ internals = { package = "bitcoin-internals", version = "0.4.0" }
 
 hashes = { package = "bitcoin_hashes", version = "0.16.0", default-features = false, optional = true }
 
+[dev-dependencies]
+hashes = { package = "bitcoin_hashes", version = "0.16.0", default-features = false, features = ["hex"] }
+
 [package.metadata.docs.rs]
 all-features = true
 rustdoc-args = ["--cfg", "docsrs"]