Stop doing borrowck crimes

[aDotInTheVoid]

    Checking skate v0.0.0 (/home/nixon/git/skate)
warning: cannot borrow `*self` as mutable because it is also borrowed as immutable
   --> src/exec.rs:173:18
    |
135 |         let function = self.functions.get(&fn_name).ok_or_else(|| {
    |                        -------------- immutable borrow occurs here
...
173 |         Ok(match self.eval_block_in(&function.body, &mut scope)? {
    |                  ^^^^                -------- immutable borrow later used here
    |                  |
    |                  mutable borrow occurs here
    |
    = note: `#[warn(mutable_borrow_reservation_conflict)]` on by default
    = warning: this borrowing pattern was not meant to be accepted, and may become a hard error in the future
    = note: for more information, see issue #59159 <https://github.com/rust-lang/rust/issues/59159>

warning: 1 warning emitted

    Finished dev [unoptimized + debuginfo] target(s) in 0.42s

rust-lang/rust#59159