[Feature request] Add exhaustive case matcher

[peterfication]

@dblock Thanks for making this gem! :)

One use case I have for enums is that I want to use them in case statements. A nice feature in Rust is exhaustive matching. Obviously, this doesn't work in native Ruby because of the missing types. However, in ruby-enum, it can work, because we have the information about which cases should be handled.

A possible drawback of this is that such a check adds additional steps to each call of the case statement and makes the runtime of the code slower.

I implemented a quick version of this in a project of mine and would be happy to contribute it to this project. A default/else block could be added as well.

Let me know what you think about it :)

##
# Adds a method to an enum class that allows for exhaustive matching on a value.
#
# @example
# class Color
# include Ruby::Enum
# include Ruby::Enum::Ecase
#
# define :RED, :red
# define :GREEN, :green
# define :BLUE, :blue
# end
#
# Color.ecase(Color::RED, {
# [Color::RED, Color::GREEN] => -> { puts "red or green" },
# Color::BLUE => -> { puts "blue" },
# })
module Ruby::Enum::Ecase
 def self.included(klass)
 klass.extend(ClassMethods)
 end

 ##
 # @see Ruby::Enum::Ecase
 module ClassMethods
 class ValuesNotDefinedError < StandardError
 end
 class NotAllCasesHandledError < StandardError
 end

 def ecase(value, cases)
 validate_cases(cases)

 cases.each do |values, block|
 values = [values] unless values.is_a?(Array)
 block.call if values.include?(value)
 end
 end

 private

 def validate_cases(cases)
 all_values = cases.keys.flatten
 superfluous_values = all_values - values
 missing_values = values - all_values

 if superfluous_values.any?
 raise ValuesNotDefinedError, "Value(s) not defined: #{superfluous_values.join(", ")}"
 end
 if missing_values.any? # rubocop:disable Style/GuardClause
 raise NotAllCasesHandledError, "Not all cases handled: #{missing_values.join(", ")}"
 end
 end
 end
end

[dblock]

I'd love this feature in Ruby::Enum! Please contribute.