Fix no_std build

This fixes the `no_std` build and also makes sure it's tested properly
in the CI workflow.

src/math.rs

@@ -0,0 +1,14 @@
+#[cfg(not(feature = "std"))]
+pub use libm::{roundf, truncf};
+
+#[cfg(feature = "std")]
+#[inline]
+pub fn roundf(x: f32) -> f32 {
+    x.round()
+}
+
+#[cfg(feature = "std")]
+#[inline]
+pub fn truncf(x: f32) -> f32 {
+    x.trunc()
+}