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1148: Add token_tree_to_xxx functions r=matklad a=edwin0cheng <del>As discus in PR #1147 , this PR added a `mbe::MacroKind` . Currently only 2 kind of macro are supported, `SourceFile` and `Block`.</del> Added following functions for `tt::TokenTree` and `ast::Node` conversion: * token_tree_to_expr * token_tree_to_pat * token_tree_to_ty * token_tree_to_macro_stmts * token_tree_to_macro_items And added two new syntax kind: * MACRO_ITEMS * MACRO_STMTS Co-authored-by: Edwin Cheng <edwin0cheng@gmail.com>