fix(actix-web/deps): clean up transitive dependencies

[AverageHelper]

Minor dependency clean-up in actix-web-helmet, as described in #12:

• Set default-features = false for actix-web (helmet only uses the macros feature, and then only while testing)
• Remove futures in favor of core::future structures.

I'm not sure if removing futures is considered "breaking" under normal circumstances, but I tested the change successfully against a local crate and needed no code changes afterward, so I figure it's fine as long as no one manually interacted with the middleware, or if they did, did not depend on the specific minor differences between futures' and Rust core's Ready.

Otherwise, simply setting default-features = false for futures would also work.

Fixes #12

[AverageHelper]

As a bonus, this crate could theoretically be made no_std as well, since as of this PR it no longer directly depends on std lib in production (edit: aside from its actix-web dep, that is 😅). It'd still need to use alloc for now tho, since it uses Box, Vec, and to_string in places.

[danielkov]

Hey @AverageHelper , thank you for your contribution (and for enabling maintainer edits). I formatted your changes with:

cargo fmt --all