This makes `background-processor` build without `std` at all. This
isn't particularly useful in the general no-std case as
`background-processor` is only useful with the `futures` feature,
and async will generally need `std` in some way or another. Still,
it ensures we don't end up reintroducing a dependency on the
current time, which breaks `wasm` use-cases.
Rust is incredibly forgiving in attempts to access `std`, making it
rather difficult to test `no-std` properly. In practice, the only
decent way to do so is to actually build for a platform that does
not have `std`, which we do here by building the `no-std-check`
crate for `arm-thumbv7m-none-eabi`.
To ensure no-std is honored across dependencies, add a crate depending
on lightning crates supporting no-std. This should ensure any
regressions are caught. Otherwise, cargo doesn't seem to catch some
incompatibilities (e.g., f64::log10 unavailable in core) and seemingly
across other dependencies as describe here:
https://blog.dbrgn.ch/2019/12/24/testing-for-no-std-compatibility/
