Distinguish slice constants from runtime slice calls

[EiNSTeiN-]

I am opening this PR to distinguish marshalled slice constants from runtime slice calls. The following expressions must retain their different meanings:

def select(data, stop):
    return data[:stop, :]


def runtime_call(data, stop):
    return data[slice(None, stop, None), :]

The normalizer currently converts any suitably shaped call named slice into colon syntax. For the single-axis case, data[slice(None, stop, None)] becomes:

data[:stop]

That removes the global lookup and call, so a shadowed or effectful slice callable no longer runs.

Conversely, a tuple containing actual serialized slice constants needs recursive normalization into colon syntax inside a multi-axis subscript. Currently data[:, :] can become:

data[(slice(None, None, None), slice(None, None, None))]

This introduces runtime global lookups that the constant-index source does not perform.

The replacement normalizes genuine PyObject::Slice values and tuples containing them, while preserving runtime calls. Variations include data[:stop, :], data[:, :], data[1:stop:2, :, 7], and single- or multi-axis runtime slice arguments.

Serialized slice objects are supported by marshal format 5, introduced in Python 3.14. Preserving actual runtime calls also matters for older bytecode; it does not require serialized slice constants.