hashes: Do not enable core2/alloc feature

`core2` is for Read/Write, nothing to do with allocation and we do not
use the "alloc" feature of `core2` in `hashes`.

Fix core2 dependency/features by doing:

- Explicitly enable "bitcoin_hashes/core2" in `bitcoin`.
- Do not enable "core2/alloc" in `hashes`

bitcoin/Cargo.toml

@@ -28,7 +28,7 @@ bitcoinconsensus-std = ["bitcoinconsensus/std", "std"]
 # Instead no-std enables additional features required for this crate to be usable without std.
 # As a result, both can be enabled without conflict.
 std = ["secp256k1/std", "bitcoin_hashes/std", "bech32/std", "bitcoin-internals/std"]
-no-std = ["core2/alloc", "bitcoin_hashes/alloc", "secp256k1/alloc"]
+no-std = ["core2/alloc", "bitcoin_hashes/alloc", "bitcoin_hashes/core2", "secp256k1/alloc"]
 
 [package.metadata.docs.rs]
 features = [ "std", "secp-recovery", "base64", "rand", "serde", "bitcoinconsensus" ]

hashes/Cargo.toml

@@ -19,13 +19,12 @@ schemars = ["actual-schemars", "dyn-clone"]
 # If you disable std, you can still use a Write trait via the core2 feature.
 # You can also use ToHex via the alloc feature, as it requires Vec/String.
 # And you can still just disable std by disabling default features, without enabling these two.
-alloc = ["core2/alloc", "internals/alloc"]
+alloc = ["internals/alloc"]
 serde-std = ["serde/std"]
 
 [dependencies]
 # Only enable this if you explicitly do not want to use "std", otherwise enable "serde-std".
 serde = { version = "1.0", default-features = false, optional = true }
-# Only enable this if you explicitly do not want to use an allocator, otherwise enable "alloc".
 core2 = { version = "0.3.0", optional = true, default_features = false }
 # TODO: change to proper version before release
 internals = { path = "../internals", package = "bitcoin-internals" }