mut deref, notice alpha renaming needed

Cargo.lock

examples/borrow.rs

examples/borrow2.rs

examples/borrow_err.rs

examples/borrow_err2.rs

examples/deref.rs

examples/deref2.rs

examples/deref3.rs

examples/deref_assign3.rs

src/borrow.rs

src/check.rs

src/env.rs

src/grammar.lalrpop

src/vm.rs

tests/test_borrow.rs

tests/test_vm.rs