Add support for AIS type 8 binary subtype of DAC=593 and FID=46

[ianfixes]

Sample data:

!AIVDM,1,1,,B,8=hQ9<fDKWPEU0,4*56
!AIVDM,1,1,,B,8=hQ9<fDKWPEU0,4*56
!AIVDM,1,1,,A,8=hQ9<fDKWPEU0,4*55
!AIVDM,1,1,,A,8=hQ9<fDKWPEU0,4*55