Implementing a verifier function to verify integrity of AST nodes

[isidentical]

BPO	40483
Nosy	@serhiy-storchaka, @pablogsal, @isidentical
Dependencies	bpo-39981: Default values for AST Nodes

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields: ```python assignee = None closed_at = None created_at = labels = ['type-feature', 'library'] title = 'Implementing a verifier function to verify integrity of AST nodes' updated_at = user = 'https://github.com/isidentical' ``` bugs.python.org fields: ```python activity = actor = 'BTaskaya' assignee = 'none' closed = False closed_date = None closer = None components = ['Library (Lib)'] creation = creator = 'BTaskaya' dependencies = ['39981'] files = [] hgrepos = [] issue_num = 40483 keywords = [] message_count = 1.0 messages = ['367969'] nosy_count = 3.0 nosy_names = ['serhiy.storchaka', 'pablogsal', 'BTaskaya'] pr_nums = [] priority = 'normal' resolution = None stage = None status = 'open' superseder = None type = 'enhancement' url = 'https://bugs.python.org/issue40483' versions = [] ```

[isidentical]

This is actually a problem that is existing for a long time. A recent example would be the discussion in the python/cpython#64067.

Implementation

• It should check existence of all fields (like ast.Constant() wont verified)
• It should check types of all fields against their declarations in ASDL (this is actually depending on python/cpython#63231) (like ast.List({ast.Constant(1)}, ctx=ast.Load()) wont pass, or the example in python/cpython#64067)

After bpo-39981 resolved, I'll submit a patch.