gdb/continuations: use lambdas instead of function pointers

gdb/continuations: use lambdas instead of function pointers

Use lambdas and std::list to track inferior continuations.  This is a
refactoring.

gdb/ChangeLog:
2021-04-22  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* inferior.h (class inferior) <continuations>: Change the type
	to be an std::list of std::function's.
	Update the references and uses below.
	* continuations.c (struct continuation): Delete.
	(make_continuation): Delete.
	(do_my_continuations_1): Delete.
	(do_my_continuations): Delete.
	(discard_my_continuations_1): Delete.
	(discard_my_continuations): Delete.
	(add_inferior_continuation): Update.
	(do_all_inferior_continuations): Update.
	(discard_all_inferior_continuations): Update.
	* continuations.h (add_inferior_continuation): Update to take
	an std::function as the parameter.
	* infcmd.c (struct attach_command_continuation_args): Delete.
	(attach_command_continuation): Delete.
	(attach_command_continuation_free_args): Delete.
	(attach_command): Update.
	(notice_new_inferior): Update.