Add lightning-background-processor no-std feature

While this is generally uneccessary as users set the `no-std` or
`std` features on the `lightning` crate directly, having this
allows `lightning-background-processor` to be built by itself
without extra dep lines. Specifically, the bindings are moving to
using the `-Z avoid-dev-deps` option, which now causes
`lightning-background-processor` to fail to build directly.

Author: TheBlueMatt

lightning-background-processor/Cargo.toml

@@ -15,7 +15,8 @@ rustdoc-args = ["--cfg", "docsrs"]
 
 [features]
 futures = [ ]
-std = ["lightning/std", "lightning-rapid-gossip-sync/std"]
+std = ["bitcoin/std", "lightning/std", "lightning-rapid-gossip-sync/std"]
+no-std = ["bitcoin/no-std", "lightning/no-std", "lightning-rapid-gossip-sync/no-std"]
 
 default = ["std"]