Rename EagerThunk to DTask

[jpsamaroo]

EagerThunk is a terrible name - let's change this to something a bit more easily explainable. Also adds a better docstring for Dagger.@spawn to help explain how it differs from Julia's Task.

Also deprecates delayed - this API is no longer recommended for various reasons, and will be removed as a user-facing API in the future.

[JamesWrigley]

Big fan of both of these changes :) I think we can go ahead and bump the version number already since this will be breaking. Another thing I noticed is that a lot of the mentioned symbols in the docstrings could be converted into linked references with e.g. [`Dagger.@spawn()`](@ref), but I can fix those later.

[JamesWrigley]

The array allocation test failure on 1.10 is a little concerning 🤔 Is this the first time it happened or is that just a flaky test?

[jpsamaroo]

The array allocation test failure on 1.10 is a little concerning 🤔 Is this the first time it happened or is that just a flaky test?

Just a flaky test! I guess it's possible to get 0 from rand() - I should probably test that there are less than 5% of zeros or something like that.