fix(no_std): replace std::string with alloc::string for compatibility

Author: Dmenec

crates/chain/src/indexer/keychain_txout.rs

@@ -10,7 +10,7 @@ use crate::{
     spk_txout::SpkTxOutIndex,
     DescriptorExt, DescriptorId, Indexed, Indexer, KeychainIndexed, SpkIterator,
 };
-use alloc::{borrow::ToOwned, vec::Vec};
+use alloc::{borrow::ToOwned, string::String, string::ToString, vec::Vec};
 use bitcoin::{
     key::Secp256k1, Amount, OutPoint, Script, ScriptBuf, SignedAmount, Transaction, TxOut, Txid,
 };
@@ -980,7 +980,7 @@ fn short_descriptor(
     edge_len: usize,
 ) -> std::string::String {
     let s = std::string::ToString::to_string(desc);
-
+    
     // +3 accounts for "..." so the shortened string won't exceed original length.
     if s.len() <= edge_len * 2 + 3 {
         return s;