Previously the crate used negative reasoning to enable `std` which was
hard to understand, required the `prelude` module and wasn't really
needed because it's only needed when a crate wants to add `alloc`
feature-backwards compatibly and this crate always had the feature.
This cleans up usage to unconditionally use `#[no_std]` and then just
add `extern crate` on top as needed by activated features.
Using the crate without allocation was previously disabled making the
crate empty without the feature. This chage makes it more fine-grained:
it only disables string and float conversions which use allocator. We
could later provide float conversions by using a sufficiently-long
`ArrayString`.
Martin Habovstiak
Tobin C. Harding