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

compiler/renderer.nim

@@ -1051,7 +1051,6 @@ proc gsub(g: var TSrcGen, n: PNode, c: TContext) =
   of nkWhileStmt: gwhile(g, n)
   of nkPragmaBlock: gpragmaBlock(g, n)
   of nkCaseStmt, nkRecCase: gcase(g, n)
-  of nkMacroStmt: gmacro(g, n)
   of nkTryStmt: gtry(g, n)
   of nkForStmt, nkParForStmt: gfor(g, n)
   of nkBlockStmt, nkBlockExpr: gblock(g, n)