Replace H256 with the one in rust-numext

Cargo.toml

@@ -11,8 +11,7 @@ exclude = ["/fixtures", "/proptest-regressions"]
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [features]
-default = ["std", "blake2b"]
-std = []
+default = ["blake2b"]
 blake2b = ["blake2b-rs"]
 
 [dependencies]

src/error.rs

@@ -1,4 +1,5 @@
 use crate::{string, H256};
+extern crate std;
 
 pub type Result<T> = ::core::result::Result<T, Error>;
 
@@ -66,5 +67,4 @@ impl core::fmt::Display for Error {
     }
 }
 
-#[cfg(feature = "std")]
 impl std::error::Error for Error {}

src/tests/tree.rs

@@ -6,6 +6,9 @@ use crate::{
 use proptest::prelude::*;
 use rand::prelude::{Rng, SliceRandom};
 
+extern crate std;
+use std::{println, vec::Vec};
+
 type SMT = SparseMerkleTree<Blake2bHasher, H256, DefaultStore<H256>>;
 
 #[test]