Feature: Preserve hash strructure when generating a TOML file

For a given hash, preserve it's structure when generating a TOML file via TOML::Generator.

Such that:
```ruby
doc = {
  "z" => 1,
  "y" => 2,
  "x" => 3
}
```
becomes:
```toml
z = 1
y = 2
x = 3
```

Current behavior is to sort the hash's keys before generating the TOML.