feat: relax FutureExt trait bound to extend IntoFuture impls instead

[nullstalgia]

Howdy!

This is a bit of an extension of #26, but more general.

Currently FutureExt only extends types that impl Future, allowing you to call .block_on() instead of .await on those types.

However this does not work for types that happen to impl IntoFuture instead, despite them also being capable of being .await-ed typically... Additionally, in the documentation for IntoFuture, it describes how calling .await desugars into a IntoFuture::into_future invocation followed by the usual poll()-ing to completion.

As such, all impl Future types also impl IntoFuture by the nature of the blanket impl within std/core, impl<F: Future> IntoFuture for F.

As far as I can tell, this change shouldn't break anything due to the above, including the MSRV (and as an aside, this also adds the MSRV to Cargo.toml).

--

My other intention with this PR is to potentially trigger a new release to crates.io, since the current unsafe-less version is quite nice, but is not currently available for mass consumption without adding a git dependency.

Lemme know if you have any questions!

[zesterer]

Thanks! I had thought this crate was feature-complete, but this is certainly a great addition.

I think I'm going to use this as a point to draw an arbitrary line in the sand and release this as 1.0.