UnsafeStrToBytes is currently not safe for -d=checkptr=2, since when it
cast from smaller struct (string) to bigger struct ([]byte). That causes
checkptr complains as the casting straddle multiple heap objects.
To fix this, we have to get the string header first, then use its fields
to construct the slice.
New implementation performs the same speed with the old (wrong) one.
name old time/op new time/op delta
UnsafeStrToBytes-8 25.7ns ± 1% 25.7ns ± 3% ~ (p=0.931 n=10+17)
name old alloc/op new alloc/op delta
UnsafeStrToBytes-8 7.00B ± 0% 7.00B ± 0% ~ (all equal)
name old allocs/op new allocs/op delta
UnsafeStrToBytes-8 0.00 0.00 ~ (all equal)
While at it, also simplify UnsafeBytesToStr implementation, since when
we can pass the slice directly to unsafe.Pointer, instead of getting the
slice header first.
