perf(ast): `ThinVec`

[Boshen]

From https://nnethercote.github.io/2023/03/24/how-to-speed-up-the-rust-compiler-in-march-2023.html

#104754: In this PR I changed a lot of Vec occurrences in AST nodes to ThinVec, which stores the length and capacity of non-empty vectors on the heap next to the vector’s elements. This means that the size of a ThinVec struct is just one word, compared to three words for Vec. This makes it a good choice for vectors that are often empty, and it can also be used to shrink the largest variants of an enum, if those variants contains a Vec. This change reduced the size of many AST nodes

I wonder how hard it is to crate our own ThinkVec based on bumpalo 🤔

[Boshen]

I really like this benchmark, I need to study how to make this:
rust-lang/rust#104754 (comment)

[Boshen]

Closing due to being too complicated for us to implement with bumpalo.