Implements #2154.

When unpacking tuples in var/let declarations a part of the tuple can now
be discarded using a single underscore.

compiler/lookups.nim

@@ -20,6 +20,7 @@ proc considerQuotedIdent*(n: PNode): PIdent =
   case n.kind
   of nkIdent: result = n.ident
   of nkSym: result = n.sym.name
+  of nkUnderscore: result = getIdent"_"
   of nkAccQuoted:
     case n.len
     of 0: