[corlib]: Misc updates from CoreFX. (#8614)
* [corlib]: Misc updates from CoreFX.
* CancellationTokenRegistration: use the CoreFX version.
* String: add `implicit operator String (ReadOnlySpan<char> value)`.
* TimeSpan: add `TimeSpan operator / (TimeSpan timeSpan, double divisor)`
and `double operator /(TimeSpan t1, TimeSpan t2)`.
* BitConverter: add `uint ToUInt32(ReadOnlySpan<byte> value)` and
`ulong ToUInt64(ReadOnlySpan<byte> value)`.
* Fix the tests.
* Bump API snapshot submodule
* [csproj] Update project files