Get the unique corresponding to the string

Property definitions

symbol :: symbol $ String :: to_symbol
	# Get the unique corresponding to the string
	fun to_symbol: Symbol
	do
		var symbol_dictionary = once new HashMap[String, Symbol]
		if symbol_dictionary.has_key(self) then
			return symbol_dictionary[self]
		else
			var symbol = new Symbol(self)
			symbol_dictionary[self] = symbol
			return symbol
		end
	end
lib/symbol/symbol.nit:17,2--28,4