Fix zh-cn translation of Chapter 8

[immccn123]

In this pull request:

1. Change 编译时间限制 into 编译时限制
   This change is made because "编译时间限制" is ambiguous, potentially leading to misunderstandings related to time, while "编译时限制" more accurately refers to constraints during the compilation process.
2. Change 特征/特性 into trait: This change is made because in Simplified Chinese, "特征" and "特性" are typically not translated into Chinese terminology. In Chapter 7, trait is not translated.
3. Change 向量 into vector: The reason is similar to the second one.
4. Fix missing translation of onto the heap in English (in L167-L168).
5. Enhanced some translations. Some translations are not clear enough (and some of them look like machine translation), which can lead to confusion for the reader.