Improved result handling for idle and timeout in execution.el
* execution.el (execution-result-timer): renamed
  `execution-result-idle' -> `execution-result-timer'
  (execution-result-idle::initialize-instance): removed
  (execution-result-idle::future-done-p): removed
  (execution-result-idle::future-get): actually respect timeout
  argument
  (execution-result-idle::future-cancel): cosmetic changes
  (execution-result-timeout): removed
  (execution-result-timeout::initialize-instance): removed
  (execution-result-timeout::future-done-p): removed
  (execution-result-timeout::future-get): removed
  (execution-result-timeout::future-cancel): removed
  (tasks-execution-make-timer-lambda): new function; construct a
  lambda expression suitable for use as time function
  (execution-idle::execution-execute): use
  `tasks-execution-make-timer-lambda'
  (execution-idle::execution-timeout): likewise