Optimized internal one-arity curry function.

@private
@category Function
@param {Function} fn The function to curry.
@return {Function} The curried function.
function _curry1(fn) {
	  return function f1(a) {
	    if (arguments.length === 0 || _isPlaceholder(a)) {
	      return f1;
	    } else {
	      return fn.apply(this, arguments);
	    }
	  };
	}