perf: eliminate allocations in scanner and formatter

[wibobm]

1. Scanning (SetString):
   • Time: ~15.7% faster.
   • Memory: Allocations were completely eliminated. It went from 32 B/op (1 allocation) down to 0 B/op (0 allocations).
2. Formatting (String()):
   • Time: ~17.3% faster.
   • Memory: Saved 48 bytes and exactly 1 allocation per operation (from 5 allocs to 4 allocs).
3. Formatting Large Zeros (fmt.Sprintf("%.100f", &z)):
   • Time: ~6.5% faster.
   • Memory: Saved 48 bytes and 1 allocation per operation.

package decimal_test

import (
	"fmt"
	"testing"

	"github.com/ericlagergren/decimal"
)

func BenchmarkScan(b *testing.B) {
	s := "123456789.987654321E+123"
	b.ResetTimer()
	b.ReportAllocs()
	for i := 0; i < b.N; i++ {
		var z decimal.Big
		z.SetString(s)
	}
}

func BenchmarkFormat(b *testing.B) {
	s := "123456789.987654321E+123"
	var z decimal.Big
	z.SetString(s)
	b.ResetTimer()
	b.ReportAllocs()
	for i := 0; i < b.N; i++ {
		_ = z.String()
	}
}

func BenchmarkFormatLargeZeros(b *testing.B) {
	s := "1.23"
	var z decimal.Big
	z.SetString(s)
	b.ResetTimer()
	b.ReportAllocs()
	for i := 0; i < b.N; i++ {
		_ = fmt.Sprintf("%.100f", &z)
	}
}