[ruby-core:116940] [Ruby master Feature#20300] Hash: set value and get pre-existing value in one call

["AMomchilov (Alexander Momchilov) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20300 has been reported by AMomchilov (Alexander Momchilov).

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Feature #20300: Hash: set value and get pre-existing value in one call
https://bugs.ruby-lang.org/issues/20300

* Author: AMomchilov (Alexander Momchilov)
* Status: Open
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When using a Hash, sometimes you want to set a new value, *and* see what was already there. Today, you **have** to do this in two steps:

```ruby
h = { k: "old value" }

# 1. Do a look-up for `:k`.
old_value = h[:k]
# 2. Do another look-up for `:k`, even though we just did that!
h[:k] = "new value"

use(old_value)
```

This requires two separate `Hash` look-ups for `:k`. This is fine for symbols, but is expensive if computing `#hash` or `#eql?` is expensive for the key. It's impossible to work around this today from pure Ruby code.

I propose adding `Hash#update_value`, which has semantics are similar to this Ruby snippet:

```ruby
class Hash
  def update_value(key, new_value)
    old_value = self[key]
    self[key] = new_value
    old_value
  end
end
```

... except it'll be implemented in C, with modifications to `tbl_update` that achieves this with a hash-lookup. 

Here's a PR with a PoC implementation: https://github.com/ruby/ruby/pull/10092



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