split again from #24038, fixes
https://github.com/status-im/nimbus-eth2/pull/6554#issuecomment-2354977102
`var`/pointer types are no longer implicitly convertible to each other
if their element types either:
* require an int conversion or another conversion operation as long as
it's not to `openarray`,
* are subtypes with pointer indirection,
Previously any conversion below a subrange match would match if the
element type wasn't a pointer type, then it would error later in
`analyseIfAddressTaken`.
Different from #24038 in that the preview define that made subrange
matches also fail to match is removed for a simpler diff so that it can
be backported.
* fixes#22054; codegen for var tuples conv
* rethink fixes
* add test cases
* templates only
* fixes var tuples
* keep varness no matter what
* fixes typ.isNil
* make it work for generics
* restore isSubrange
* add a test case as requested
