refactor: simplify AST type definition and make type erasure pass more stable

[Peefy]

1. Does this PR affect any open issues?(Y/N) and add issue references (e.g. "fix #123", "re #123".):

• N
• Y

re #970

refactor: simplify AST type definition and make type erasure pass more stable

In the current AST type definition, a mixture of NodeRef<String> and NodeRef<Type> is used. This PR removes all NodeRef<String>, making the AST definition simpler and more organized.

2. What is the scope of this PR (e.g. component or file name):

• kclvm/ast/src/ast.rs
• kclvm/ast_pretty/src/node.rs
• kclvm/compiler/src/codegen/llvm/node.rs
• kclvm/parser/src/parser/expr.rs
• kclvm/parser/src/parser/stmt.rs

3. Provide a description of the PR(e.g. more details, effects, motivations or doc link):

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Other

4. Are there any breaking changes?(Y/N) and describe the breaking changes(e.g. more details, motivations or doc link):

• N
• Y

5. Are there test cases for these changes?(Y/N) select and add more details, references or doc links:

• Unit test
• Integration test
• Benchmark (add benchmark stats below)
• Manual test (add detailed scripts or steps below)
• Other

• All parser unit test cases and integration grammar test cases.

[coveralls]

Pull Request Test Coverage Report for Build 7407967348

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-15.0%) to 72.803%

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Totals
Change from base Build 7406880388:	-15.0%
Covered Lines:	43361
Relevant Lines:	59559

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💛 - Coveralls

[zong-zhe]

LGTM