fix(base): fix index out of range when using iterator

[yingmanwumen]

I found a bug:

>>> import ulist as ul
>>> foo = ul.arange(2)
>>> iterater = iter(foo)
>>> next(iterater)
0
>>> next(iterater)
1
>>> next(iterater)
thread '<unnamed>' panicked at 'Index '2' out of range '2'!', src/base.rs:133:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/yingmanwumen/Documents/Codes/Rust/ulist/ulist/python/ulist/core.py", line 80, in __getitem__
    return self._values.get(index)
pyo3_runtime.PanicException: Index '2' out of range '2'!

This causes the following code to fail

...
foo = ul.arange(n)
for x in foo:
    # ....do something
...

I found that the use of panic instead of exception in the rust code result in it.

[tushushu]

Thanks for doing this PR. I am actually thinking of using PyErr instead of Panic and created an issue.