perf: Improve the speed of coins.String() (#10076)

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## Description

Speedup coins.String() and coin.String()

In my local benchmarks, this >2x improves the time for balances with 1 coin, and further improves speed for strings with many balances. I did not benchmark on the two coin usecase

---

### Author Checklist

*All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.*

I have...

- [x] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [x] added `!` to the type prefix if API or client breaking change
- [x] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
- [x] provided a link to the relevant issue or specification - n/a
- [x] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules)
- [x] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing) - covered by existing
- [ ] added a changelog entry to `CHANGELOG.md`
- [x] included comments for [documenting Go code](https://blog.golang.org/godoc) - seems sufficiently clear
- [x] updated the relevant documentation or specification
- [x] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed

### Reviewers Checklist

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I have...

- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed 
- [ ] reviewed state machine logic
- [ ] reviewed API design and naming
- [ ] reviewed documentation is accurate
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

CHANGELOG.md

@@ -87,6 +87,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 
 ### Improvements
 
+* (types) [\#10076](https://github.com/cosmos/cosmos-sdk/pull/10076) Significantly speedup and lower allocations for `Coins.String()`.
 * (x/bank) [\#10022](https://github.com/cosmos/cosmos-sdk/pull/10022) `BankKeeper.SendCoins` now takes less execution time.
 * (deps) [\#9956](https://github.com/cosmos/cosmos-sdk/pull/9956) Bump Tendermint to [v0.34.12](https://github.com/tendermint/tendermint/releases/tag/v0.34.12).
 * (cli) [\#9856](https://github.com/cosmos/cosmos-sdk/pull/9856) Overwrite `--sequence` and `--account-number` flags with default flag values when used with `offline=false` in `sign-batch` command.

types/coin.go

@@ -34,7 +34,7 @@ func NewInt64Coin(denom string, amount int64) Coin {
 
 // String provides a human-readable representation of a coin
 func (coin Coin) String() string {
-	return fmt.Sprintf("%v%v", coin.Amount, coin.Denom)
+	return fmt.Sprintf("%v%s", coin.Amount, coin.Denom)
 }
 
 // Validate returns an error if the Coin has a negative amount or if
@@ -185,13 +185,18 @@ func (coins Coins) MarshalJSON() ([]byte, error) {
 func (coins Coins) String() string {
 	if len(coins) == 0 {
 		return ""
+	} else if len(coins) == 1 {
+		return coins[0].String()
 	}
 
-	out := ""
-	for _, coin := range coins {
-		out += fmt.Sprintf("%v,", coin.String())
+	// Build the string with a string builder
+	var out strings.Builder
+	for _, coin := range coins[:len(coins)-1] {
+		out.WriteString(coin.String())
+		out.WriteByte(',')
 	}
-	return out[:len(out)-1]
+	out.WriteString(coins[len(coins)-1].String())
+	return out.String()
 }
 
 // Validate checks that the Coins are sorted, have positive amount, with a valid and unique