Add 'transform' operator that can be applied to various monad types

[lewissbaker]

Support applying a function that transforms value of type A to value of type B to:

• task<A> to produce task<B> (etc. for other task types)
• generator<A> to prouduce generator<B> (etc. for other generator types)

Example syntax:

B a_to_b(const A& a);
cppcoro::task<A> make_an_a();

cppcoro::task<B> b = make_an_a() | cppcoro::transform(a_to_b);

For task<T> this is kind of similar to the proposed std::future<T>::then() method.

[lewissbaker]

Commits 1871e52 and fab674f add an fmap() operator for task, lazy_task and async_generator.

[lewissbaker]

One design question is whether to always return a unique task (task or lazy_task) even when the input is a shared_task or shared_lazy_task.

If we did then the caller would need to call make_shared_task to re-promote it to a shared task at cost of an extra coroutine frame allocation.

I expect that most use-cases will not need a shared task result, however, so returning a unique task is probably going to be more efficient.

[lewissbaker]

Still need to add fmap support for generator and recursive_generator.

A similar issue arises with fmap implementation for recursive_generator as to whether the result should be a recursive_generator or just a generator.

[lewissbaker]

Commit 8e72893 adds fmap support for generator, recursive_generator and shared_task. It also adds docs to the README.

I've decided to go with following transformations when fmap is applied to specialisations of fundamental monad types:

• recursive_generator -> generator
• shared_task -> task