Use unsafe.Slice for WKB handling

[peterstace]

Go 1.17 adds a new function unsafe.Slice. Currently, WKB marshalling/unmarshalling use unsafe in a few ways that are legal, but can be make safer and less error prone with unsafe.Slice.

E.g. rather than having the following code:

var floats []float64
hdr := (*reflect.SliceHeader)(unsafe.Pointer(&floats))
hdr.Data = (*reflect.SliceHeader)(unsafe.Pointer(&byts)).Data
hdr.Len = len(byts) / 8
hdr.Cap = cap(byts) / 8
return floats

We can do something like this instead (untested):

return unsafe.Slice((*float64)(unsafe.Pointer(&byts[0])), len(byts)/8)

There may be a few other places in WKB marshalling/unmarshalling that we can cleanup using the new unsafe functions in Go 1.17.