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
```
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