added exercises

.gitignore

frac_rs/.gitignore

frac_rs/Cargo.toml

frac_rs/src/frac.rs

frac_rs/src/main.rs

pi_rs/.gitignore

pi_rs/Cargo.toml

pi_rs/src/main.rs

pi_rs/src/square.rs

queen_rs/.gitignore

queen_rs/Cargo.toml

queen_rs/src/main.rs

sorting_algos_rs/.gitignore

sorting_algos_rs/Cargo.toml

sorting_algos_rs/src/main.rs

stack_rs/.gitignore

stack_rs/Cargo.toml

stack_rs/src/main.rs

stack_rs/src/stack/mod.rs

stack_slice/.gitignore