Add :stringify_names option to convert symbol keys to string for dumping

[fnordfish]

When dumping larger and/or nested Hashes, it would be useful to let Psych treat symbol keys as string keys to avoid building a new hash with string keys, just for the sake of dumping.

:stringify_names for dumping is conceptually the reverse of :symbolize_names for loading:

yaml = "---\nfoo: bar\n"
Psych.dump(Psych.load(yaml, symbolize_names: true), stringify_names: true) == yaml # => true

[fnordfish]

Did a small benchmark, comparing symbolize_names: true vs. manual deep stringify keys: https://gist.github.com/fnordfish/4b22aed6281800897e56399d6c53c0be

While overall performance gains seem to be within margin of error, memory usage is consistently down.

[tenderlove]

This seems like a fine feature, but is the option name the right thing? It's stringify_names, but that seems pretty ambiguous from the reader's perspective. Maybe stringify_symbol_keys or something?

[fnordfish]

I'm absolutely not married to the name.

But just for the sake of the argument, symbolize_names does also only work on string keys:

Psych.safe_load("---\n!ruby/object:DateTime 2024-01-17 11:19:12.359689000 +01:00: value\nkey: value\n", symbolize_names: true, permitted_classes: [Time, DateTime])
# => {#<DateTime: 2024-01-17T11:19:12+01:00 ((2460327j,37152s,359689000n),+3600s,2299161j)>=>"value", :key=>"value"}

[tenderlove]

But just for the sake of the argument, symbolize_names does also only work on string keys:

Ya, you're right. Looking through commit history, it seems like that came from JSON.parse. I guess since we have documentation it's probably fine. I'll merge this. Thank you!