struct SegmentId: Add newtype wrapper for seg_id that enforces in-bounds

[kkysen]

• Part of fn decode_coefs is slow #1180.

It also removes some checks from fn decode_b and its callees, which is also a hot spot.

[randomPoison]

The change looks good, just had a couple of minor comments/questions. Have you confirmed that this change does actually result in removing the bounds checks, though? How did you go about verifying that?

[kkysen]

Have you confirmed that this change does actually result in removing the bounds checks, though? How did you go about verifying that?

Yes; I looked at the assembly.