scte35: create constants for all segmentation descriptor types

[ollytom]

The values are documented in SCTE 35 section SCTE 35 section 10.3.3.1. Right now we have magic values floating around. See in particular:

crc_32.go:95: 	0x30476DC0,
splice_descriptor.go:152: 		case 0x34, 0x30, 0x32, 0x36, 0x38, 0x3a, 0x44, 0x46:
splice_descriptor.go:206: 		case 0x34, 0x30, 0x32, 0x36, 0x38, 0x3a, 0x44, 0x46:
splice_info.go:17: 	SAPNone                     = 0x30

These should be constants named, for example, ProviderAdStart or so.