feat(StringList): implement str_len

[yingmanwumen]

#131
The method behaves like:

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

But I don't know how to add a test case for this implementation.
First PR for ulist ^_^

[tushushu]

#131 The method behaves like:

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

But I don't know how to add a test case for this implementation. First PR for ulist ^_^

Hey, @yingmanwumen. Thanks for the PR.
We use pytest to test the new functions and methods. For this PR, the test cases are put in ulist/tests/test_string.py. After adding your test cases, please run cd /you_path/../tests/ and pytest test_string.py locally. The PR can also trigger the Github action to run the unit tests in the cloud automatically.

[tushushu]

All looks good except the docstring, and please add some tests cases for this method. I mentioned that in another comment.

[tushushu]

Maybe """Return each element's string length of self.""" would be better:)

[yingmanwumen]

^_^ OK, I'll modify it latter. I am not at expressing something in English and I am trying to practice. Thanks, have a good day~

[tushushu]

@yingmanwumen All look good, thanks for submitting this PR.