Using Symbol#name allows to hit two birds with one stone.

First it will return a pre-existing string, so will save
one allocation per key.

Second, that string will be already interned, so it will
save the internal `Hash` implementation the work of looking
up the interned strings table to deduplicate the key.

```
ruby 3.3.0 (2023-12-25 revision 5124f9ac75) [x86_64-darwin21]
Warming up --------------------------------------
                to_s    17.768k i/100ms
                cond    23.703k i/100ms
Calculating -------------------------------------
                to_s    169.830k (±10.4%) i/s -    852.864k in   5.088377s
                cond    236.803k (± 7.9%) i/s -      1.185M in   5.040945s

Comparison:
                to_s:   169830.3 i/s
                cond:   236803.4 i/s - 1.39x  faster
```

```ruby
require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'
  gem 'benchmark-ips', require: false
end

HASH = {
  first_name: nil,
  last_name: nil,
  country: nil,
  profession: nil,
  language: nil,
  hobby: nil,
  pet: nil,
  longer_name: nil,
  occupation: nil,
  mailing_address: nil,
}.freeze

require 'benchmark/ips'

Benchmark.ips do |x|
  x.report("to_s") { HASH.transform_keys(&:to_s) }
  x.report("cond") { HASH.transform_keys { |k| Symbol === k ? k.name : k.to_s } }
  x.compare!(order: :baseline)
end
```