Depend on libm in no-std for powf(64)

In the next commits we'll need `f64`'s `powf`, which is only
available in `std`. For `no-std`, here we depend on `libm` (a
`rust-lang` org project), which we can use for `powf`.

Author: TheBlueMatt

lightning/Cargo.toml

@@ -31,7 +31,7 @@ unsafe_revoked_tx_signing = []
 # Override signing to not include randomness when generating signatures for test vectors.
 _test_vectors = []
 
-no-std = ["hashbrown", "bitcoin/no-std", "core2/alloc"]
+no-std = ["hashbrown", "bitcoin/no-std", "core2/alloc", "libm"]
 std = ["bitcoin/std"]
 
 # Generates low-r bitcoin signatures, which saves 1 byte in 50% of the cases
@@ -48,6 +48,7 @@ regex = { version = "1.5.6", optional = true }
 backtrace = { version = "0.3", optional = true }
 
 core2 = { version = "0.3.0", optional = true, default-features = false }
+libm = { version = "0.2", optional = true, default-features = false }
 
 [dev-dependencies]
 regex = "1.5.6"

lightning/src/lib.rs

@@ -69,6 +69,7 @@ extern crate hex;
 #[cfg(any(test, feature = "_test_utils"))] extern crate regex;
 
 #[cfg(not(feature = "std"))] extern crate core2;
+#[cfg(not(feature = "std"))] extern crate libm;
 
 #[cfg(ldk_bench)] extern crate criterion;
 

lightning/src/routing/scoring.rs

@@ -1469,6 +1469,16 @@ impl<G: Deref<Target = NetworkGraph<L>>, L: Deref, T: Time> ScoreUpdate for Prob
 impl<G: Deref<Target = NetworkGraph<L>>, L: Deref, T: Time> Score for ProbabilisticScorerUsingTime<G, L, T>
 where L::Target: Logger {}
 
+#[cfg(feature = "std")]
+#[inline]
+fn powf64(n: f64, exp: f64) -> f64 {
+	n.powf(exp)
+}
+#[cfg(not(feature = "std"))]
+fn powf64(n: f64, exp: f64) -> f64 {
+	libm::powf(n as f32, exp as f32) as f64
+}
+
 mod approx {
 	const BITS: u32 = 64;
 	const HIGHEST_BIT: u32 = BITS - 1;