Bind MemoryReservation::split() to Python

[madsbk]

Exposes the split() added in #1177 as MemoryReservation.split(size).

Together with the existing opaque_memory_usage context manager, callers can scope part of a reservation to the allocation it covers:

with opaque_memory_usage(reservation.split(reorder_bytes)):
    reordered, offsets = plc.partitioning.hash_partition(...)
# the reorder's bytes are released here

instead of computing the remainder and releasing it by hand. This lets cudf-polars' shuffle actors drop a helper and its bookkeeping at two call sites.

Also fixes the opaque_memory_usage docstring example, which passed an argument the function doesn't take.

[pentschev]

LGTM.

[madsbk]

Thanks @pentschev

[madsbk]

/merge