perf: make Stack::peek() return a reference

[mooori]

Modifies Stack::peek() to return a reference, i.e. &U256 instead of U256.
Since U256 derives Copy, call sites which need to own to the value returned
by peek may dereference to create a copy:

let x = *stack.peek(n);

Regarding Stack::peek_h256(): I think the H256 value must be constructed
inside the function and since it's not possible to return a reference to a local
variable, peek_h256 cannot return a reference &H256. If I'm missing
something here, please let me know!

The steps mentioned here under Reproduction can be taken to benchmark
this change in aurora-engine.

Ref: #448

[mooori]

This PR was made to save the code and result. Closing it since this change leads to a slight increase in NEAR gas usage:

thread 'tests::repro::repro_5bEgfRQ' panicked at 'assertion failed: `(left == right)`
  left: `706`,
 right: `707`', engine-tests/src/tests/repro.rs:146:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
test tests::repro::repro_5bEgfRQ ... FAILED
thread 'tests::repro::repro_FRcorNv' panicked at 'assertion failed: `(left == right)`
  left: `197`,
 right: `198`', engine-tests/src/tests/repro.rs:146:5