Auto releasing execution threads when executor is released

Please see #32 and #29

Allow task execution to throw errors

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 API usages

v0.6.5

Fix deprecated atomic APIs used in AtomicReference (#27)

Fix concurrency limit deadlock

v0.6.4

signal before recursion (#25)

Rename serial executor

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.

Allow limiting concurrency when using concurrent executor

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

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

Fix typo in task ID tracking parameter

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

Fix typo in task ID tracking parameter

Allow tracking task ID to support reporting timeout error details

When a timeout occurs while waiting for a sequence of tasks to finish execution, it is useful to understand which task was executing when the timeout occurred.

Fix SequenceExecutionHandle accessibility

Merge pull request #16 from uber/execution-handle_accessibility

Fix SequenceExecutionHandle accessibility

Fix SerialSequenceExecutor accessibility

Merge pull request #15 from uber/execution-serial_executor

Fix SerialSequenceExecutor accessibility