StringMethods should have the same methods as standard str
Should `StringMethods` be compatible with standard `str`, or have only useful methods? Sometimes I have to adjust text formats before converting other types, and I've chosen additionals which makes formatting / adjustments easier. Any ideas?
- [x] `str.find` (#9386)
- [x] `str.rfind` (#9386)
- [x] `str.isalnum` (#9282)
- [x] `str.isalpha` (#9282)
- [x] `str.isdigit` (#9282)
- [x] `str.isspace` (#9282)
- [x] `str.islower` (#9282)
- [x] `str.isupper` (#9282)
- [x] `str.istitle` (#9282)
- [x] `str.ljust` (#9352)
- [x] `str.rjust` (#9352): `StringMethods.pad` works as the same as `ljust`, `rjust` and `center` by switching `side` option. And `StringMethods.center` exists, `ljust` and `rjust` don't.
- [x] `str.rsplit`: `split` also supports regex, and there seems no straightforward way to `rsplit` using regex. (#10303)
- [x] `str.zfill` (#9387)
- [x] `StringMethods.center` and `StringMethods.pad` should have 'fillchar` kw to specify padding character.  (#9352)
- [x] `str.capitalize` (#9766)
- [x] `str.swapcase` (#9766)
- [x] `str.translate` (#10052)
- [x] `str.index` (#10045)
- [x] `str.rindex` (#10045)
- [x] `str.partition` (#9773)
- [x] `str.rpartition` (#9773)
- [x] `unicode.isnumeric` (#9439)
- [x] `unicode.isdecimal` (#9439)
- [x] `unicodedata.normalize` (#10031)

**NOTE** For python 2.x, once convert target to unicode internally then call the func.    
## Which may not be very userful:
- `expandtabs`
- `splitlines`

I think there is an option to implement these to say "support the same methods as standard `str`".
