Disable default features for optional zeroize dependency

We don't make use of the `zeroize` implmeentation for `Vec`, so we can
just drop its dependency on `alloc`, which is enabled by default.

Author: tnull

Cargo.toml

@@ -46,7 +46,7 @@ crate_rand = { package = "rand", version = ">=0.6.0, <0.9.0", optional = true }
 serde = { version = "1.0", default-features = false, features = [ "alloc" ], optional = true }
 
 # Enabling this feature raises the MSRV to 1.51
-zeroize = { version = "1.5", features = ["zeroize_derive"], optional = true }
+zeroize = { version = "1.5", features = ["zeroize_derive"], default-features = false, optional = true }
 
 # Unexported dependnecies
 bitcoin_hashes = { version = ">=0.12, <=0.13", default-features = false }