`AllImplicitConversionTargets` replaces `ImplicitConversionTargets`

The function `ImplicitConversionTargets` in module `std.traits` has a
design flaw: The list of conversion targets contains some, but not all
unsafe conversion targets. To overcome this, a new function
`AllImplicitConversionTargets` has been added and
`ImplicitConversionTargets` has been deprecated.
