Update ConcurrentSequenceExecutor to use AutoReleasingSemaphore (#33)

* Update ConcurrentSequenceExecutor to use AutoReleasingSemaphore

Fixes #29

* Remove debugging code

Allow task execution to throw errors (#28)

* Allow task execution to throw errors

Adds support to allow task execution to throw errors. The error is then reported back via the `SequenceExecutionHandle.await` method when results are being retrieved.

* Ignore SPM generated scheme

* Xcode project changes

Fix deprecated atomic APIs used in AtomicReference (#27)

signal before recursion (#25)

Rename SerialSequenceExecutor (#23)

Renames `SerialSequenceExecutor` to `ImmediateSerialSequenceExecutor` to better represent the execution is done immediately on the caller thread, since a `ConcurrentSequenceExecutor` limited to a maximum concurrency level of 1 would also be serial.

Avoid using semaphore if there is no concurrency limit (#22)

Avoid paying the cost of calling the semaphore if no limit is set.

Merge pull request #20 from uber/execution-fix_typo-master

Fix typo in task ID tracking parameter

Merge pull request #19 from uber/executor-track_task_id-master

Allow tracking task ID to support reporting timeout error details

Merge pull request #16 from uber/execution-handle_accessibility

Fix SequenceExecutionHandle accessibility

Merge pull request #15 from uber/execution-serial_executor

Fix SerialSequenceExecutor accessibility