Implement `str_len` method for `StringList`

[tushushu]

Need to implement StringList.str_len method to return a IntegerList object with the length of each string.

[yingmanwumen]

Does this method behave like this:

>>> import ulist as ul
>>> strs = ul.from_seq(["hello", "world!"], dtype='string')
>>> strs.str_len()
UltraFastList([5, 6])

[tushushu]

Does this method behave like this:

>>> import ulist as ul
>>> strs = ul.from_seq(["hello", "world!"], dtype='string')
>>> strs.str_len()
UltraFastList([5, 6])

Exactly, the dtype of the result should be int64 by the way. Please feel free to submit a PR if you are interested.