no-std compatiblity for underlying traits

[TTWNO]

Hi there,

First of all, thank you for Tower. So many cool projects have come from it!

To be clear: this PR does not make the main tower crate, nor its Filter, Make, Steer, etc. no_std compatible. This is not the goal, nor do I expect this from the tower team.

I'd like to be able to use the base tower-service and tower-layer crates in an embedded context—in this case, one which does not have a standard library. While the code in tower-layer and tower-service are simple (and permissively licensed enough) to just copy over, it'd be nice to share in the ecosystem, and occasionally publish reusable no_std compatible services when I can.

This PR:

• makes tower-service and tower-layer no_std compatible,
• adds a active-by-default std feature to tower-service in order to optionally link the external alloc crate for the Box implementation, and
• makes tower-layer no_std compatible unconditionally, as it does not depend on any alloc features.

Please let me know if this is something you'd consider.

[TTWNO]

Let me know if there are other/more/less places this should be noted in the documentation.