[SizzleUnrlsd]

Summary

First item of #71: the three syntax forms that rejected whole files on the large projects measured in #69.

• Acceptance tests first (test: define assignment expressions, yield from and starred targets), implementation follows.
• Assignment expressions. New HIR node NamedExpr; the scope analysis binds the target in the enclosing function, also from inside a comprehension as PEP 572 specifies; the CFG builder lays the walrus out as an assignment before the statement that reads it, and a while (line := f()): condition is recomputed every iteration like the other control-flow conditions.
• yield from. Yield gains a delegate flag; the adapter builds it instead of failing; lowering is unchanged.
• Starred assignment targets. A tuple target may hold Starred; lowering gives the starred target the slice the others leave and indexes the targets after it from the end; scopes, the CFG builder and the reassigned-parameter scan follow the starred value.
• Two tests that encoded the old limits now use forms that stay unsupported (async comprehensions, exception groups); the usage guide lists the three forms.
• Regression snapshots: healthchecks goes from 636 to 653 files analysed (2 566 to 2 746 functions) and yt-fts to 20/20 files; the 17 syntax-error notes are replaced by the findings of those files.

Part of #71.