Prevent Numeric#to_s from allocating an array

`benchmark/ips` don't show any significant difference:

```ruby
Benchmark.ips do |x|
  x.report('default_to_s') { 42.to_default_s }
  x.report('as_to_s') { 42.to_s }
end
```

before the patch:

```
        default_to_s    34.094k i/100ms
             as_to_s    30.675k i/100ms
-------------------------------------------------
        default_to_s      4.916M (± 9.0%) i/s -     24.241M
             as_to_s      2.382M (± 9.2%) i/s -     11.749M
```

after the patch:

```
Calculating -------------------------------------
        default_to_s    37.690k i/100ms
             as_to_s    34.579k i/100ms
-------------------------------------------------
        default_to_s      5.024M (±11.1%) i/s -     24.536M
             as_to_s      3.294M (± 6.3%) i/s -     16.390M
```

That being said the standard deviation is pretty high, and `MemoryProfiler` don't seems to agree that splat args allocate an array. In any case splat args are slightly more costly than regular arguments especially since https://bugs.ruby-lang.org/issues/9622
BigDecimal default argument value is different too.