Remove no_std support from SDK

[matthiasgoergens]

@codeblooded1729 says:

That makes me wonder, do we even need no_std if we can get away with restricted_std.

@matthiasgoergens says:

When someone doesn't use std functionality, ideally the dead code elimination optimisation pass would take care of it.

So let's see whether we can get rid of no_std support in our sdk, and perhaps simplify a lot of conditional compilation. (Or otherwise, learn why we can't get rid of it.)