chore: create "lib.rs" & update ".gitignore"

Author: duguorong009

.gitignore

@@ -1,6 +1,6 @@
 # Generated by Cargo
 # will have compiled files and executables
-/target/
+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

core/src/lib.rs

@@ -0,0 +1 @@
+pub mod number;