Optimize BigDecimal#to_s

[byroot]

Most of the time was spent in two calls to snprintf, by a simpler integer to ASCII function, it can be made several time faster.

The code is largely adapted from an earlier version of ruby/json. ruby/json now use a much more optimized algorithm, but there are licensing consideration so not sure it's worth optimizing that much.

Before:

ruby 4.0.2 (2026-03-17 revision d3da9fec82) +YJIT +PRISM [arm64-darwin25]
Warming up --------------------------------------
               22.99   408.215k i/100ms
               large   230.802k i/100ms
Calculating -------------------------------------
               22.99      4.214M (± 2.2%) i/s  (237.32 ns/i) -     21.227M in   5.040152s
               large      2.384M (± 2.6%) i/s  (419.45 ns/i) -     12.002M in   5.037698s

After:

ruby 4.0.2 (2026-03-17 revision d3da9fec82) +YJIT +PRISM [arm64-darwin25]
Warming up --------------------------------------
               22.99     1.026M i/100ms
               large   846.057k i/100ms
Calculating -------------------------------------
               22.99     10.882M (± 0.8%) i/s   (91.89 ns/i) -     55.426M in   5.093603s
               large      9.094M (± 1.0%) i/s  (109.97 ns/i) -     45.687M in   5.024549s

require "bundler/inline"

gemfile do
  source 'https://rubygems.org'
  gem "benchmark-ips"
  gem "bigdecimal", path: "/Users/byroot/src/github.com/byroot/bigdecimal"
end

small = BigDecimal("29.99")
large = BigDecimal("32423094234234.23423432")
Benchmark.ips do |x|
  x.report("22.99") { small.to_s }
  x.report("large") { large.to_s }
end

[mrkn]

@byroot Thanks a lot.