Add functions to decode vast and vmap using xmlTokenizer

[CarlLindqvist]

Could probably be faster, but this will handle things coming in as CDATA or just straight xml encoded

goos: darwin
goarch: arm64
pkg: github.com/Eyevinn/VMAP/vmap
cpu: Apple M2 Pro
BenchmarkUnmarshal-12       	    1389	    862222 ns/op	  280116 B/op	    6786 allocs/op
BenchmarkFasterDecode-12    	    7166	    162484 ns/op	   60466 B/op	     868 allocs/op
PASS
ok  	github.com/Eyevinn/VMAP/vmap	3.654s

[Lunkers]

While I personally feel like some variable names aren't the most intuitive, It is standard practice for golang; that nitpick aside, this looks good to me!