Introduce exec::function<...>

[ispeters]

This PR proposes a new type-erased sender named exec::function. There's an in-code comment giving a bunch of examples, but a simple example is:

exec::function<int(int)> fn(42, [](int i) { return ex::just(i); });

auto [result] = ex::sync_wait(std::move(fn)).value();

assert(result == 42);

There are a bunch of TODOs left, including lots of tests that are missing, but the API is ready to collect early feedback. If this looks like a promising direction, I intend to write a paper for Brno proposing this type for inclusion in C++29.