add 闭环

[flaneur2020]

• [x] add token about the 闭环 and lexer
• [x] add parser about the 闭环 statement
• [x] refactor: take reference instead of move on iterating the ast nodes
• [x] add the interpreter logic about the 闭环 statement
• [ ] support 破圈(break) statement (maybe we could implement it in another PR)

[chenhengqi]

BTW, please add some unit tests for this feature.

[flaneur2020]

BTW, please add some unit tests for this feature.

thank your for remind, i've add some test cases about this, please check again 🤝

[chenhengqi]

LGTM