VM: support importc var, ptr/pointer types, cast int <=> ptr/pointer (#12877)

* VM: allow certain hardcoded special var variables at CT
* VM: allow all importc var, cast[int](ptr)
* fix tests tests/vm/tstaticprintseq.nim, tests/cpp/t8241.nim
* VM: == works for ptr/pointer nodes
* bugfix: ==, cast now also works for pointer, not just ptr
* VM supports cast PtrLikeKinds <=> PtrLikeKinds / int
* improve cname handling
* fixup + bug fix
* VM: support cast from ref to int
* address comment: opcLdGlobalDeref => opcLdGlobalDerefFFI
* defensive check against typ == nil

compiler/vmdef.nim

@@ -63,6 +63,8 @@ type
     opcCastIntToFloat64,    # int and float must be of the same byte size
     opcCastFloatToInt32,    # int and float must be of the same byte size
     opcCastFloatToInt64,    # int and float must be of the same byte size
+    opcCastPtrToInt,
+    opcCastIntToPtr,
     opcFastAsgnComplex,
     opcNodeToReg,
 
@@ -169,6 +171,8 @@ type
     opcAsgnConst, # dest = copy(constants[Bx])
     opcLdGlobal,  # dest = globals[Bx]
     opcLdGlobalAddr, # dest = addr(globals[Bx])
+    opcLdGlobalDerefFFI, # dest = globals[Bx][]
+    opcLdGlobalAddrDerefFFI, # globals[Bx][] = ...
 
     opcLdImmInt,  # dest = immediate value
     opcNBindSym, opcNDynBindSym,