Fix no_std build

Author: yukibtc

Makefile

@@ -1,7 +1,7 @@
 precommit:
 	cargo fmt --all -- --config format_code_in_doc_comments=true
-	cargo clippy --all && cargo clippy --all --no-default-features
-	cargo test --all && cargo test --all --no-default-features
+	cargo clippy -p negentropy && cargo clippy -p negentropy --no-default-features
+	cargo test -p negentropy && cargo test -p negentropy --no-default-features
 
 bench:
 	RUSTFLAGS='--cfg=bench' cargo +nightly bench -p negentropy

negentropy/src/lib.rs

@@ -18,6 +18,7 @@ extern crate std;
 #[cfg(not(feature = "std"))]
 use alloc::collections::BTreeSet;
 use alloc::collections::VecDeque;
+use alloc::string::String;
 use alloc::vec;
 use alloc::vec::Vec;
 use core::cmp::Ordering;

negentropy/src/sha256.rs

@@ -1,6 +1,8 @@
 // Copyright (c) 2023 Yuki Kishimoto
 // Distributed under the MIT software license
 
+use alloc::vec::Vec;
+
 const K: [u32; 64] = [
     0x428a2f98, 0x71374491, 0xb5c0fbcf, 0xe9b5dba5, 0x3956c25b, 0x59f111f1, 0x923f82a4, 0xab1c5ed5,
     0xd807aa98, 0x12835b01, 0x243185be, 0x550c7dc3, 0x72be5d74, 0x80deb1fe, 0x9bdc06a7, 0xc19bf174,