Replace tfHasRequiresInit with a more accurate mechanism

The new mechanism can deal with more complex scenarios such as
not nil field appearing in a non-default case object branch or
a field within a generic object that may depend on a when branch.

The commit also plugs another hole: the user is no longer able
to create illegal default values through seq.setLen(N).

compiler/semtypinst.nim

@@ -613,6 +613,8 @@ proc replaceTypeVarsTAux(cl: var TReplTypeVars, t: PType): PType =
 
       of tyObject, tyTuple:
         propagateFieldFlags(result, result.n)
+        if result.kind == tyObject and cl.c.computeRequiresInit(cl.c, result):
+          result.flags.incl tfRequiresInit
 
       of tyProc:
         eraseVoidParams(result)