fix #4556

This implements a number of new safety checks and error messages
when object constructors are used:

In case objects:

* the compiler will prevent you from initializing fields in
conflicting branches

* When a field from a particular branch is initialized, the
compiler will demand that the discriminator field is also
supplied with a maching compile-time value

In all objects:

* When the "requiresInit" pragma is applied to a type, all fields
of the type must be initialized when object construction is used.

The code will be simplified in a follow up commit.

compiler/semdata.nim

@@ -45,7 +45,7 @@ type
     inst*: PInstantiation
 
   TExprFlag* = enum
-    efLValue, efWantIterator, efInTypeof,
+    efLValue, efWantIterator, efWantStatic, efInTypeof,
     efWantStmt, efAllowStmt, efDetermineType, efExplain,
     efAllowDestructor, efWantValue, efOperand, efNoSemCheck,
     efNoProcvarCheck, efNoEvaluateGeneric, efInCall, efFromHlo,