fix(stdlib): builtins.map multi-iterable binding + add coverage for untested builtins

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Summary

Adds tests for builtins that were bound but had no coverage, and fixes a binding bug uncovered by the new tests.

🐞 Bug fix

builtins.map 2- and 3-iterable overloads — the function parameter was typed as 'T1 * 'T2 -> 'T3 (tuple-input), which Fable compiles to a Python function expecting a single tuple argument. But Python's map(f, it1, it2) calls f(a, b) positionally, so the overloads failed at runtime with TypeError: takes 1 positional argument but 2 were given. Switched both overloads to System.Func<...>, matching the pattern already used by functools.reduce and itertools.accumulate. Call sites use the natural curried form: builtins.map ((fun a b -> a + b), xs, ys).

🧪 Tests

Adds 17 new [<Fact>] tests covering builtins that had zero coverage:

Function	What's tested
abs	int and float absolute values
chr / ord	ASCII + non-BMP round-trip (☃)
len	string length and (Python-native) list length
map	1-, 2-, and 3-iterable overloads
str	converts int and bool to string
int	string parse, float truncation toward zero (incl. negative)
float	string parse and int conversion
isinstance	true/false using pyInt/pyStr type references with emitPyExpr
type	returns a usable type object
bytes	from byte array and from string with encoding (length and byte content)

isinstance/type tests use emitPyExpr to construct genuine Python primitives (per the comment in the existing pyInstanceof test), since F# int/float/bool compile to Fable wrapper classes.

Notes

• Rebased onto current main to pick up the type references and pyInt/pyStr helpers from feat(stdlib): expose Python type references and dict/list constructors in Builtins #291.
• No CHANGELOG.md entry — assumed to be auto-generated from commit messages.

CI will validate that the Fable-compiled Python passes all assertions.