Consider using `Box` on type parameters to reduce statement allocation size

In https://github.com/RustPython/Parser/pull/93, we add type_params: Vec to ClassDef and FunctionDef which increases the size of a statement. We may want to optimize this field to reduce the size of the statement struct.

The main reason for the assertion is to be aware of how changing the AST changes its size. This is important because ruff and RustPython hold many instances of these types and increasing their size mean:

• Each element in a Vec now requires more storage -> Overall increased memory consumption
• Reading or writing an element becomes slower because your computer must read or write more bytes. This is especially relevant for reading where the L1 cache short circuits reads by caching about 64 bytes of neighboring memory to avoid subsequent reads from (the very slow) memory.

There's not much we can do about this here. This could be a place where using tinyvec over a regular Vec would be beneficial, considering that the vec will be empty for almost all instances. But I think this is fine for now.

Originally posted by @MichaReiser in https://github.com/RustPython/Parser/pull/93#discussion_r1259220991

首先查看 PR #93 以及 ClassDef 和 FunctionDef 的 AST 定义，重点关注新增的 type_params 字段和讨论中描述的语句大小断言。比较当前 Vec 与提议的 Box 方案对分配的影响，同时考虑提到的 tinyvec 替代方案；当所选表示能够减少不必要的语句大小且不破坏 AST 时，工作就完成了。