Use '0'+n for converting single digit to char

[tompng]

In both case (snprintf, Vp_ulltoa), nn is always a single digit number in range 0..9.

[tompng]

Benchmark

require "bundler/inline"

gemfile do
  source 'https://rubygems.org'
  gem "benchmark-ips"
  gem "bigdecimal" # Use `-Ilib` to load the local version of bigdecimal
end

small = BigDecimal("29.99")
large = BigDecimal("32423094234234.23423432")
huge = BigDecimal('9' * 1000 + '.' + '9' * 1000)
Benchmark.ips do |x|
  x.report("22.99") { small.to_s }
  x.report("large") { large.to_s }
  x.report("huge") { huge.to_s }
end

Before

ruby 4.0.0 (2025-12-25 revision 553f1675f3) +YJIT +MN +PRISM [arm64-darwin24]
Warming up --------------------------------------
               22.99   809.036k i/100ms
               large   670.072k i/100ms
                huge    19.506k i/100ms
Calculating -------------------------------------
               22.99      9.116M (± 1.5%) i/s  (109.70 ns/i) -     46.115M in   5.059908s
               large      7.640M (± 1.3%) i/s  (130.88 ns/i) -     38.864M in   5.087452s
                huge    199.845k (± 1.8%) i/s    (5.00 μs/i) -      1.014M in   5.077167s

After

ruby 4.0.0 (2025-12-25 revision 553f1675f3) +YJIT +MN +PRISM [arm64-darwin24]
Warming up --------------------------------------
               22.99   995.850k i/100ms
               large   980.671k i/100ms
                huge    70.413k i/100ms
Calculating -------------------------------------
               22.99     10.766M (± 2.4%) i/s   (92.88 ns/i) -     54.772M in   5.090304s
               large     11.430M (± 1.7%) i/s   (87.49 ns/i) -     57.860M in   5.063603s
                huge    700.051k (± 3.1%) i/s    (1.43 μs/i) -      3.521M in   5.033985s