Deprecate and remove 'eager' task classes `task` and `shared_task` about cppcoro

[lewissbaker]

Started the process of removing use of task<T> from tests in b78bc3e.

There are still the where_all_tests.cpp and where_all_ready_tests.cpp to go but I've been running into some crashes when updating these that I need to investigate.

[lewissbaker]

Commit 722fe1b removes use of eager tasks from where_all_ready_tests.cpp.

Only where_all_tests.cpp to go.

[lewissbaker]

Commit 421de76 removes last of eager task usage from tests (except where testing eager tasks themselves).

[lewissbaker]

Commit 64b0a04 removes the task<T> and shared_task<T> classes and updates the README.

All that remains now is renaming lazy_task -> task and shared_lazy_task -> shared_task.

[lewissbaker]

Commit bf9eb2c has implemented the rename.

Closing.