def register_macro(name, assertion_macro)
        macros[name.to_s] = assertion_macro
      end
Registers a [Riot::AssertionMacro] class to a given +name+. Name is distinct, which means any future
registrations for the same name will replace previous ones. +name+ will always be converted to a
string first.

@param [String, Symbol] name The handle the macro will be associated with
@param [Class] assertion_macro A [Riot::AssertionMacro] class