Fix memoize keys for required keyword arguments

[jakezwang]

memoize assumes that required parameters are passed positionally. A unary call such as memoized(x=1) raises IndexError, while calls to a binary function with different keyword arguments can share the empty-tuple cache key and return the wrong result.

Bind supplied keywords to the function signature before constructing the optimized cache key. Positional calls retain their existing keys, and equivalent positional and keyword calls share cached results.

Tests cover unary and binary functions, mixed and reordered arguments, cache reuse, and invalid keywords. The full test and doctest suites, benchmarks, style checks, and changed-file pre-commit hooks pass; tests and benchmarks also pass on Python 3.9, 3.14 and 3.15.