Build with stable rust 1.75

This updates to latest git embassy deps (soon to be published on
crates.io).

The two nightly rust features are no longer required. Async Fn in Traits
has been merged.

Type Alias Impl Trait isn't in stable yet, so uses of singleton!() need
to be replaced with more cumbersome StaticCell. Eventually that can be
reverted.

There are also some small API changes to embassy and embedded-io crates
that needed fixing.