Add custom I/O executor support to I/O objects.

All I/O objects now have an additional Executor template parameter. This
template parameter defaults to the asio::executor type (the polymorphic
executor wrapper) but can be used to specify a user-defined executor
type.

I/O objects' constructors and functions that previously took an
asio::io_context& now accept either an Executor or a reference to a
concrete ExecutionContext (such as asio::io_context or
asio::thread_pool).

One potential point of breakage in existing user code is when reusing an
I/O object's io_context for constructing another I/O object, as in:

    asio::steady_timer my_timer(my_socket.get_executor().context());

To fix this, either construct the second I/O object using the first I/O
object's executor:

    asio::steady_timer my_timer(my_socket.get_executor());

or otherwise explicitly pass the io_context:

    asio::steady_timer my_timer(my_io_context);

asio/include/Makefile.am

@@ -8,6 +8,8 @@ nobase_include_HEADERS = \
 	asio/basic_io_object.hpp \
 	asio/basic_raw_socket.hpp \
 	asio/basic_seq_packet_socket.hpp \
+	asio/basic_serial_port.hpp \
+	asio/basic_signal_set.hpp \
 	asio/basic_socket_acceptor.hpp \
 	asio/basic_socket.hpp \
 	asio/basic_socket_iostream.hpp \
@@ -51,6 +53,7 @@ nobase_include_HEADERS = \
 	asio/detail/cstddef.hpp \
 	asio/detail/cstdint.hpp \
 	asio/detail/date_time_fwd.hpp \
+	asio/detail/deadline_timer_service.hpp \
 	asio/detail/dependent_type.hpp \
 	asio/detail/descriptor_ops.hpp \
 	asio/detail/descriptor_read_op.hpp \
@@ -128,6 +131,7 @@ nobase_include_HEADERS = \
 	asio/detail/impl/win_thread.ipp \
 	asio/detail/impl/win_tss_ptr.ipp \
 	asio/detail/io_control.hpp \
+	asio/detail/io_object_executor.hpp \
 	asio/detail/io_object_impl.hpp \
 	asio/detail/is_buffer_sequence.hpp \
 	asio/detail/is_executor.hpp \
@@ -390,6 +394,8 @@ nobase_include_HEADERS = \
 	asio/local/stream_protocol.hpp \
 	asio/packaged_task.hpp \
 	asio/placeholders.hpp \
+	asio/posix/basic_descriptor.hpp \
+	asio/posix/basic_stream_descriptor.hpp \
 	asio/posix/descriptor_base.hpp \
 	asio/posix/descriptor.hpp \
 	asio/posix/stream_descriptor.hpp \
@@ -453,6 +459,10 @@ nobase_include_HEADERS = \
 	asio/uses_executor.hpp \
 	asio/version.hpp \
 	asio/wait_traits.hpp \
+	asio/windows/basic_object_handle.hpp \
+	asio/windows/basic_overlapped_handle.hpp \
+	asio/windows/basic_random_access_handle.hpp \
+	asio/windows/basic_stream_handle.hpp \
 	asio/windows/object_handle.hpp \
 	asio/windows/overlapped_handle.hpp \
 	asio/windows/overlapped_ptr.hpp \

asio/include/asio.hpp

@@ -23,6 +23,9 @@
 #include "asio/basic_io_object.hpp"
 #include "asio/basic_raw_socket.hpp"
 #include "asio/basic_seq_packet_socket.hpp"
+#include "asio/basic_serial_port.hpp"
+#include "asio/basic_signal_set.hpp"
+#include "asio/basic_socket.hpp"
 #include "asio/basic_socket_acceptor.hpp"
 #include "asio/basic_socket_iostream.hpp"
 #include "asio/basic_socket_streambuf.hpp"
@@ -94,6 +97,8 @@
 #include "asio/local/stream_protocol.hpp"
 #include "asio/packaged_task.hpp"
 #include "asio/placeholders.hpp"
+#include "asio/posix/basic_descriptor.hpp"
+#include "asio/posix/basic_stream_descriptor.hpp"
 #include "asio/posix/descriptor.hpp"
 #include "asio/posix/descriptor_base.hpp"
 #include "asio/posix/stream_descriptor.hpp"
@@ -119,6 +124,10 @@
 #include "asio/uses_executor.hpp"
 #include "asio/version.hpp"
 #include "asio/wait_traits.hpp"
+#include "asio/windows/basic_object_handle.hpp"
+#include "asio/windows/basic_overlapped_handle.hpp"
+#include "asio/windows/basic_random_access_handle.hpp"
+#include "asio/windows/basic_stream_handle.hpp"
 #include "asio/windows/object_handle.hpp"
 #include "asio/windows/overlapped_handle.hpp"
 #include "asio/windows/overlapped_ptr.hpp"