Check for no-std compatibility across dependencies

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/

Author: jkczyz

.github/workflows/build.yml

@@ -122,6 +122,10 @@ jobs:
           cargo test --verbose --color always --no-default-features --features no-std
           # check if there is a conflict between no-std and the default std feature
           cargo test --verbose --color always --features no-std
+          # check no-std compatibility across dependencies
+          cd ..
+          cd no-std-check
+          cargo check --verbose --color always
           cd ..
       - name: Test on no-std builds Rust ${{ matrix.toolchain }} and full code-linking for coverage generation
         if: "matrix.build-no-std && matrix.coverage"

.gitignore

@@ -8,3 +8,4 @@ lightning-c-bindings/a.out
 Cargo.lock
 .idea
 lightning/target
+no-std-check/target

Cargo.toml

@@ -9,6 +9,10 @@ members = [
     "lightning-background-processor",
 ]
 
+exclude = [
+    "no-std-check",
+]
+
 # Our tests do actual crypo and lots of work, the tradeoff for -O1 is well worth it.
 # Ideally we would only do this in profile.test, but profile.test only applies to
 # the test binary, not dependencies, which means most of the critical code still

no-std-check/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "no-std-check"
+version = "0.1.0"
+edition = "2018"
+
+[features]
+default = ["lightning/no-std", "lightning-invoice/no-std"]
+
+[dependencies]
+lightning = { version = "0.0.105", path = "../lightning", default-features = false }
+lightning-invoice = { version = "0.13.0", path = "../lightning-invoice", default-features = false }