Rewrite unsafeStringToBytes to satisfy 1.16 vet check (#466)

Fixes #465.

According to https://golang.org/pkg/unsafe/#Pointer (6), one of the only safe ways to use pointers:
> Conversion of a reflect.SliceHeader or reflect.StringHeader Data field to or from Pointer.
...
> In general, reflect.SliceHeader and reflect.StringHeader should be used only as *reflect.SliceHeader and *reflect.StringHeader pointing at actual slices or strings, never as plain structs. A program should not declare or allocate variables of these struct types.

This rewrites the unsafe to only modify to not allocate a `reflect.SliceHeader ` manually making the vet check happy.

wire/unsafe.go

@@ -29,13 +29,14 @@ import (
 // new memory for it with the assumption that the resulting byte slice will not
 // be mutated.
 func unsafeStringToBytes(s string) []byte {
-	sh := (*reflect.StringHeader)(unsafe.Pointer(&s))
-	sliceHeader := reflect.SliceHeader{
-		Data: sh.Data,
-		Len:  sh.Len,
-		Cap:  sh.Len,
-	}
-	return *(*[]byte)(unsafe.Pointer(&sliceHeader))
+	p := unsafe.Pointer((*reflect.StringHeader)(unsafe.Pointer(&s)).Data)
+
+	var b []byte
+	hdr := (*reflect.SliceHeader)(unsafe.Pointer(&b))
+	hdr.Data = uintptr(p)
+	hdr.Cap = len(s)
+	hdr.Len = len(s)
+	return b
 }
 
 // unsafeBytesToString converts a byte slice into a string without allocating