Regression: local variables cleared in case of runtime errors

[sharkdp]

>>> let x = 1

  let x: Scalar = 1

>>> 1/0
error: runtime error
 = Division by zero

>>> x
thread 'main' panicked at numbat/src/vm.rs:567:41:
index out of bounds: the len is 0 but the index is 79
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This is because we now keep local variables on the stack (since #197), and we clear the stack when encountering a runtime error here:

numbat/numbat/src/vm.rs

Line 505 in 69653b0

self.stack.clear();