attribute: avoid reflection for small fixed-array slice types in hashKV

[alliasgher]

Fixes #8200.

Applies the same type-switch fast path used for SLICE in #8166 to the four existing slice types: BOOLSLICE, INT64SLICE, FLOAT64SLICE, and STRINGSLICE.

The slice values for len 0-3 are stored as [N]T fixed arrays by SliceValue. Type-asserting directly on those avoids all reflection overhead for the most common lengths. Longer slices fall through to the existing reflect.ValueOf path unchanged.

Benchmarks

BenchmarkHashKVsSlices exercises only slice-type attributes (len 2-3, which hit the fixed-array fast path):

BenchmarkHashKVsSlices-10    ~99 ns/op    0 B/op    0 allocs/op

Micro-benchmark of the per-element path for [2]bool:

old (reflect): ~7 ns/op
new (type switch): ~1 ns/op   (~5.7x faster)

The overall BenchmarkHashKVs is unchanged since it includes many non-slice types, but the slice-specific path is significantly faster.