TypedPython: fixed-layout classes (SPEC N-11, M2b) #123

Compile classes whose layout is fixed in CPython too (user-written __slots__ equal to the annotated fields, base object, trivial __init__): slot reads/writes, allocate-and-fill construction, is/None, class-typed parameters. Same results as the interpreter, including when the class or its module global changes after init.

Done when: binary-trees' make and check compile with Node declaring __slots__, outputs agree with CPython/Node/Java/Rust, the TypedPython suite passes, and the class-change scenarios (property installed after init, global rebinding, mid-body changes, watcher exhaustion) equal the interpreted module. Benchmark records CPython with and without __slots__.