Could `Duration` receive methods to directly conve...

# stdlib

[Derek Peirce]

Could Duration receive methods to directly convert to and from its underlying Long, similar to `Double`'s toBits and fromBits? I'd like to avoid boxing and take advantage of AndroidX's primitive collections, but creating a value class MutableDurationList(private val longs: MutableLongList) requirese a 1:1 conversion, and converting directly to/from a specific unit is both lossy and inefficient compared to using the stored Long.