This is mainly to simplify the code and use less memory, as large hash can use quite a lot:

```ruby
>> ObjectSpace.memsize_of(1000.times.map { |i| [i, i]}.to_h)
=> 28768
>> ObjectSpace.memsize_of(10_000.times.map { |i| [i, i]}.to_h)
=> 458848
```

The performance is mostly not impacted, if not slightly better:

```ruby
require 'benchmark/ips'
require 'active_support/all'
@store = ActiveSupport::Cache::MemoryStore.new
@store.write("small", "small")
Benchmark.ips do |x|
  x.report("read:miss") { @store.read("miss") }
  x.report("read:small") { @store.read("small") }
  x.report("write:small") { @store.write("small", "small") }
end
```

6.0.3.2:
```
Warming up --------------------------------------
           read:miss    42.466k i/100ms
          read:small    25.315k i/100ms
         write:small    17.826k i/100ms
Calculating -------------------------------------
           read:miss    426.923k (± 1.9%) i/s -      2.166M in   5.074890s
          read:small    248.518k (± 2.7%) i/s -      1.266M in   5.097049s
         write:small    180.388k (± 1.6%) i/s -    909.126k in   5.041238s
```

This branch:
```
Warming up --------------------------------------
           read:miss    42.040k i/100ms
          read:small    28.364k i/100ms
         write:small    19.361k i/100ms
Calculating -------------------------------------
           read:miss    417.814k (± 2.1%) i/s -      2.102M in   5.033186s
          read:small    278.950k (± 2.8%) i/s -      1.418M in   5.088135s
         write:small    193.384k (± 1.8%) i/s -    968.050k in   5.007446s
```