Fix no-std builds

coins/monero/generators/README.md

@@ -4,4 +4,4 @@ Generators used by Monero in both its Pedersen commitments and Bulletproofs(+).
 An implementation of Monero's `ge_fromfe_frombytes_vartime`, simply called
 `hash_to_point` here, is included, as needed to generate generators.
 
-This library is usable under no_std when the `alloc` feature is enabled.
+This library is usable under no-std when the `std` feature is disabled.

coins/monero/src/wallet/scan.rs

@@ -1,6 +1,7 @@
 use core::ops::Deref;
 use std_shims::{
   vec::Vec,
+  string::ToString,
   io::{self, Read, Write},
 };