FoR will compress signed array when min == 0 now

[a10y]

Fixes #509

Previously, FoR would bail out early if it detected that min == 0. However, we still want to FoR encode when the array is signed, so that we get a bitpackable unsigned array out of FoR compression

[a10y]

Referencing the example from the ticket, here's the previous encoding of the timestmap_col:

  "timestamp_col": vortex.datetimeparts(0x16)(ext(vortex.timestamp, ExtMetadata([2, 0, 0])), len=65536) nbytes=524.29 kB (0.02%)
    metadata: DateTimePartsMetadata { days_dtype: Primitive(I64, NonNullable), seconds_dtype: Primitive(I64, NonNullable), subseconds_dtype: Primitive(I64, NonNullable) }
    days: vortex.constant(0x0a)(i64, len=65536) nbytes=0 B (0.00%)
      metadata: ConstantMetadata { scalar: Scalar { dtype: Primitive(I64, NonNullable), value: Primitive(I64(0)) }, length: 65536 }
    seconds: vortex.primitive(0x03)(i64, len=65536) nbytes=524.29 kB (0.02%)
      metadata: PrimitiveMetadata { validity: NonNullable }
      buffer: 524.29 kB
    subsecond: vortex.constant(0x0a)(i64, len=65536) nbytes=0 B (0.00%)
      metadata: ConstantMetadata { scalar: Scalar { dtype: Primitive(I64, NonNullable), value: Primitive(I64(0)) }, length: 65536 }

And with this change, FoR + bitpacking gets applied:

  "timestamp_col": vortex.datetimeparts(0x16)(ext(vortex.timestamp, ExtMetadata([2, 0, 0])), len=65536) nbytes=131.07 kB (0.01%)
    metadata: DateTimePartsMetadata { days_dtype: Primitive(I64, NonNullable), seconds_dtype: Primitive(I64, NonNullable), subseconds_dtype: Primitive(I64, NonNullable) }
    days: vortex.constant(0x0a)(i64, len=65536) nbytes=0 B (0.00%)
      metadata: ConstantMetadata { scalar: Scalar { dtype: Primitive(I64, NonNullable), value: Primitive(I64(0)) }, length: 65536 }
    seconds: fastlanes.for(0x0f)(i64, len=65536) nbytes=131.07 kB (0.01%)
      metadata: FoRMetadata { reference: Scalar { dtype: Primitive(I64, NonNullable), value: Primitive(I64(0)) }, shift: 0 }
      encoded: fastlanes.bitpacked(0x0e)(u64, len=65536) nbytes=131.07 kB (0.01%)
        metadata: BitPackedMetadata { validity: NonNullable, patches_len: 0, bit_width: 16, offset: 0, length: 65536 }
        packed: vortex.primitive(0x03)(u64, len=16384) nbytes=131.07 kB (0.01%)
          metadata: PrimitiveMetadata { validity: NonNullable }
          buffer: 131.07 kB
    subsecond: vortex.constant(0x0a)(i64, len=65536) nbytes=0 B (0.00%)
      metadata: ConstantMetadata { scalar: Scalar { dtype: Primitive(I64, NonNullable), value: Primitive(I64(0)) }, length: 65536 }