Add math mod for lib

math/.gitignore

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+/target
+**/*.rs.bk
+Cargo.lock
+bin/
+pkg/
+wasm-pack.log
+.cargo-ok

math/Cargo.toml

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+[package]
+name = "math"
+version = "0.1.0"
+authors = ["Sambo Chea <sombochea@cubetiqs.com>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

math/src/lib.rs

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+mod math {
+    #[allow(dead_code)]
+    pub fn sum(a: i32, b: i32) -> i32 {
+        a + b
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn it_works() {
+        assert_eq!(math::sum(10, 10), 20);
+    }
+}