Draft: Split stable feature into std+stable
Add a new std feature, selected by default. This controls whether the
lib is built for no_std and uses features from std.
stable is not used to control whether nightly features are used. The
const_fn_trait_bound feature will be stable in Rust 1.61.0.
Allows building for no_std by disabling default features.
Allows building on stable channel by selecting the stable feature.
On x86, PIO does not use std and writing to /dev/port, and instead
always uses the IN and OUT assembly instructions.