Don't include default features of slab

[emil-e]

slab includes the std feature by default which won't work for no_std projects. I can't really reproduce the problem by just building the project by itself but when pulling in futures-concurrency in my no_std/embassy project, I get errors such as:

cannot find type `Option` in this scope
not found in this scope   

Fix is simply to exclude default features from slab. Seems to work even when std features is enabled for futures-concurrency.