Add factorial examples in Rust

03_recursion/rust/03_factorial/.gitignore

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+# Generated by Cargo
+# will have compiled files and executables
+/target/
+
+# Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
+# More information here https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html
+Cargo.lock
+
+# These are backup files generated by rustfmt
+**/*.rs.bk

03_recursion/rust/03_factorial/Cargo.toml

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+[package]
+name = "factorial"
+version = "0.1.0"
+authors = ["Alexander Launi <alex.launi@gmail.com>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

03_recursion/rust/03_factorial/src/main.rs

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+fn fact(x: u32) -> u32 {
+    if x == 1 {
+        return 1;
+    }
+
+    return x * fact(x - 1);
+}
+
+fn main() {
+    println!("{}", fact(3));
+}