Move to a no_std Implementation

[appcypher]

Apart from intrinsic memory allocation needed for collections, smart pointers, etc., the library does not depend on a lot of system functions that the OS provides therefore making it a good candidate for a no_std implementation with just the alloc and collection features enabled.

There is really no pressing need for this now and requirements might change as the implementation evolves. But it would be nice to enforce this non-dependency somehow so this issue is here for that as well as other needs for it in the future.

[zeeshanlakhani]

Thanks for writing this up @appcypher.

[matheus23]

As of last week, async_once_cell (which we're using) has support for no_std.