Remove `Utils::Inflector` and `Utils::String.pluralize` / `.singularize`

[Luca Guidi]

From 2.0 Hanami will use dry-inflector instead of Utils::Inflector.

• Remove Utils::Inflector
• Remove Utils::String.pluralize and .singularize

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This change MUST go to unstable branch of this repository.

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Ref https://trello.com/c/nJhd5awo

[Oana Sipos]

Hey @artofhuman! Saw you submitted a PR regarding this issue, may I assign it to you?

[Semyon Pupkov]

@oana-sipos yes ;)

[Luca Guidi]

Closed by #319