Change supertype of <signed integer type>, UInt64 from Float64 to Int128

[nameexhaustion]

Description

It has historically been upcasted to Float64 as we did not have an Int128 type:

polars/crates/polars-core/src/utils/supertype.rs

Line 234 in e950592

(Int64, UInt64) => Some(Float64), // Follow numpy

For bigidx especially, we currently upcast to Int64 which may cause some data loss in the UInt64 column:

polars/crates/polars-core/src/utils/supertype.rs

Lines 235 to 236 in e950592

	#[cfg(feature = "bigidx")]
	(Int64, UInt64) => Some(Int64), // Needed for bigidx