Idea: add Char type to represent Text with size = 1

该 issue 未指定文件、测试或入口点；请先审阅 issue 中提出的 Char 和 Text 语义。完成的标准是形成一份达成共识的设计，涵盖可迭代行为、索引和切片、兼容性以及拼接规则。

Due to the lack of character type in Python, a function that accept List[Text] will need to be extra careful when generalize the accepted argument type. Iterable[Text] and Sequence[Text] clearly doesn't work:

In [1]: import collections.abc

In [2]: isinstance("foo", collections.abc.Sequence)
Out[2]: True

In [3]: isinstance("foo", collections.abc.Iterable)
Out[3]: True

The closest thing you can do is to do something like Union[List[Text], Set[Text], Tuple[Text]] but it will not work with things like KeysView.

How about having a typing.Char to represent Text with length=1, and dedicate Text to a type with at least 2 characters?

Some examples to illustrate the usage:

def foo(xs: Iterable[Text]):
  return '|'.join(xs)

# list(Text) should return List[Char]
foo("abcd") # error
foo(list("abcd")) # correct, List[Char] is a special case of List[Text]

def bar(xs: Iterable[Char]):
  return '|'.join(xs)

bar("abcd") # correct, Text is a Iterable[Char]
bar(list("abcd")) # correct, List[Char] is a Iterable[Char]
bar(["ab", "cd"]) # error, List[Text] is not a Iterable[Char]

a: Char = 'a'

a[1] # error, Char cannot be indexed beyond index 0
a[0] # should be an error from type checking's PoV, Char should never be indexed
a[:] # should be an error from type checking's PoV, Char should never be copied as a slice

b: Text = 'b'
a + b # should be an error from type checking's PoV, Char and Text should be considered incompatible types
typing.cast(Text, a) + b # correct

c = a + a # correct, concatenate Char gets Text (but I'm not sure if this is reasonable though, what if we do (a + a) + a ?)