LDMD: Made error message on compiler execution error clearer.

It is also triggered when the compiler exits with a signal, etc.
DValue: Do not uselessly inherit from Object.
DValue: struct->class.

This is in line with an incoming upstream change making all
polymorphic structs classes, and also matches common coding
conventions.
Minor DVarValue refactoring.
Detab RTTIBuilder.
Downgraded accepts-invalid workaround error to assertion.

Bug 9268 was fixed in 2.062.
Store bools as i8.

I really hope we can refactor this to use a less leaky
abstraction later – it should at least be possible to merge
voidToI8 and i1ToI8.