Add a lower-level `Params::Types::Map` type

[postmodern]

Add a lower-level Params::Types::Map type, which Params::Types::Enum will inherit from. This type will allow defining an arbitrary mapping of String values to other Ruby objects:

param :arch, Map[
  'x86-64' => :x86_64,
  ...
], desc: 'The architecture to compile for'