Support `logger::Record` in C by String-ing the fmt::Arguments

Support `logger::Record` in C by String-ing the fmt::Arguments

This adds a new (non-feature) cfg argument `c_bindings` which will
be set when building C bindings. With this, we can (slightly) tweak
behavior and API based on whether we are being built for Rust or C
users.

Ideally we'd never need this, but as long as we can keep the API
consistent-enough to avoid material code drift, this gives us a
cheap way of doing the "right" thing for both C and Rust when the
two are in tension.

We also move lightning-background-processor to support the same
MSRV as the main lightning crate, instead of only
lightning-net-tokio's MSRV.