[spec] conceptual foundation and glossary entries for variance

Currently the section on variance says:

A longer introduction to these concepts can be found on Wikipedia and in PEP 483; here we just show how to control a type checker’s behavior.

Now that we have a core concepts section in the spec, it would be good for the spec to also derive the rationale for variance from those core concepts, rather than deferring to the (ten-year-old and not updatable) PEP 483.

We should also add glossary entries for variance terminology: invariant, covariant, contravariant, and variance.

先从规范中关于型变的章节和现有的核心概念章节开始，然后比较 PEP 483 以及链接的 Wikipedia 文章中的依据。完成的标准是：型变章节从核心概念推导出其依据，并且术语表定义 invariant、covariant、contravariant 和 variance。