incrementalmerkletree: Remove `std` from default features.

`std` was actually only ever required for `test-dependencies`, which is
not in `default`.

incrementalmerkletree/Cargo.toml

@@ -30,7 +30,9 @@ rand_core = "0.6"
 rand_chacha = "0.3"
 
 [features]
-default = ["std"]
+# The `std` feature is no longer enabled by default because it is not needed
+# for backwards compatibility; it is only enabled by the `test-dependencies`
+# feature.
 std = []
 # The legacy-api feature guards types and functions that were previously
 # part of the `zcash_primitives` crate. Those types were removed in the

incrementalmerkletree/src/frontier.rs

@@ -681,8 +681,8 @@ pub mod testing {
     use proptest::prelude::*;
     use rand::{distributions::Standard, prelude::Distribution};
 
-    use std::collections::hash_map::DefaultHasher;
-    use std::hash::Hasher;
+    #[cfg(feature = "std")]
+    use {core::hash::Hasher, std::collections::hash_map::DefaultHasher};
 
     use crate::{frontier::Frontier, Hashable, Level};
 
@@ -701,6 +701,7 @@ pub mod testing {
     #[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
     pub struct TestNode(pub u64);
 
+    #[cfg(feature = "std")]
     impl Hashable for TestNode {
         fn empty_leaf() -> Self {
             Self(0)