syntax compatibility between do blocks and stmt blocks

See the section `do notation` in the manual for more info.

* nkMacroStmt has been removed
   Macro statements are now mapped to regular nkCall nodes.
   The support for additional clauses (such as else, except, of, etc)
   have been restored - they will now appear as additional arguments
   for the nkCall node (as nkElse, nkExcept, etc nodes)

* fixed some regressions in the `is` operator and semCompiles

tests/run/tmacros1.nim

@@ -1,5 +1,5 @@
 discard """
-  output: "Got: 'nnkMacroStmt' hi"
+  output: "Got: 'nnkCall' hi"
 """
 
 import
@@ -11,17 +11,15 @@ macro outterMacro*(n: stmt): stmt {.immediate.} =
   proc innerProc(i: int): string =
     echo "Using arg ! " & n.repr
     result = "Got: '" & $n.kind & "' " & $j
-  if n.kind != TNimrodNodeKind.nnkMacroStmt:
-    error("Macro " & n[0].repr & " requires a block.")
   var callNode = n[0]
-  expectKind(callNode, TNimrodNodeKind.nnkCall)
-  if callNode.len != 2 or callNode[1].kind != TNimrodNodeKind.nnkIdent:
+  expectKind(n, TNimrodNodeKind.nnkCall)
+  if n.len != 3 or n[1].kind != TNimrodNodeKind.nnkIdent:
     error("Macro " & callNode.repr &
       " requires the ident passed as parameter (eg: " & callNode.repr & 
       "(the_name_you_want)): statements.")
   result = newNimNode(TNimrodNodeKind.nnkStmtList)
-  var ass : PNimrodNode = newNimNode(TNimrodNodeKind.nnkAsgn)
-  ass.add(newIdentNode(callNode[1].ident))
+  var ass : PNimrodNode = newNimNode(nnkAsgn)
+  ass.add(newIdentNode(n[1].ident))
   ass.add(newStrLitNode(innerProc(4)))
   result.add(ass)