genSym support for hygienic macros and templates.

example:
template hygienic(val: expr) =
  var `*x` = val
  echo `*x`

*x was chosen as mnemonic for "opposite of public" and thus private

compiler/semexprs.nim

@@ -13,7 +13,7 @@
 proc semTemplateExpr(c: PContext, n: PNode, s: PSym, semCheck = true): PNode = 
   markUsed(n, s)
   pushInfoContext(n.info)
-  result = evalTemplate(c, n, s)
+  result = evalTemplate(n, s)
   if semCheck: result = semAfterMacroCall(c, result, s)
   popInfoContext()