Complete the PyHIR node set #25

PyHIR is the parser-independent representation every semantic analysis depends on.

Covered: Module, Function, Class, Assign, AugAssign, Return, Name, Constant, Call, Attribute, Subscript, BinaryOp, UnaryOp, BoolOp, Compare, Tuple, List, Dict, Comprehension, If, While, For, Break, Continue, Raise, With, Assert, Import, ImportFrom, Global, Nonlocal, Pass. Still diagnostics: Lambda, except*, yield from, star arguments, nested function definitions.

• Class (feat: control-flow graphs #12), with decorators (feat: everyday syntax coverage and tolerant check #26)
• AugAssign (feat: everyday syntax coverage and tolerant check #26)
• If, While, For (feat: control-flow graphs #12)
• Await, Yield (feat: try with exception edges, await and yield #36)
• Raise (feat: control-flow graphs #12), With (feat: everyday syntax coverage and tolerant check #26), Try with exception edges (feat: try with exception edges, await and yield #36)
• Comprehension (feat: package layout alignment and lexical scope analysis #4); Lambda still a diagnostic
• Chained comparisons, keyword arguments, decorators, defaults (feat: everyday syntax coverage and tolerant check #26); star arguments still a diagnostic
• PyHIR schema version (HIR_SCHEMA_VERSION, feat: everyday syntax coverage and tolerant check #26)

Reference: docs/architecture.md §3.2 PyHIR.

Delivery: #26, #36