Introduce no_std

[hoijui]

To be honest, I think this makes little sense for this library.
I did it, because I never implemented it before, and wanted to try.

I also did something that is discouraged:
Having a no_std feature which has to be manually enabled
whenever feature std is disabled.
This allows to not compile dependency tinyvec when
feature std is enabled.
The alternative would be, to remove the optional = true
from tinyvec in Cargo.toml,
remove feature no_std,
and live with tinyvec being compiled in the std case,
even though it is not used there.

I can imagine this would be ok to have around
as a separate branch in this repo,
but would probably not merge it into master.

[becheran]

😎 in general I like the idea. But as you already wrote I don't think there are a lot of people wanting this. But who knows. If more people would comment on this pr I am not fully against it.