Fix a bug introduced by the parens fix

  The fix for #81 ignored a case where a layout block was started by ParenL
symbol.  In that case, the layout level wouldn't be pushed on the stack, and a
parse error would eventually result.  This adds the check into the ParenL
special case, starting an explicit block terminated by a ParenR, as well as a
new virtual layout block.

  Also missing from the original patch was the full offsides check, which must
be repeated in the ParenL special case.  I think that we should probably do some
refactoring of the layout processor, as I doubt that this is the only case that
will produce this behavior.

  Additionally, the BraceL/BraceR cases were removed, as explicitly delimited
layout is not currently supported.

src/Cryptol/Parser/LexerUtils.hs

@@ -236,22 +236,16 @@ layout cfg ts0
     -- If we find the EOF, we close all open blocks, and then we stop.
     | EOF   <- ty = extra ++ [ virt cfg (to pos) VCurlyR | _ <- stack ] ++ [t]
 
-    -- Left parens and braces start new explicit blocks
-    | Sym ParenL <- ty = t : loop False (Explicit (Sym ParenR) : stack) ts
-    | Sym CurlyL <- ty = t : loop False (Explicit (Sym CurlyR) : stack) ts
+    -- Left parens start new explicit blocks
+    | Sym ParenL <- ty = t : loop False (Explicit (Sym ParenR) : parensStack) ts
 
-    -- Right parens and braces close to the nearest explicit block, failing if
-    -- they don't properly close it
+    -- Right parens close to the nearest explicit block, failing if they don't
+    -- properly close it
     | Sym ParenR <- ty
     , Explicit (Sym ParenR) : ps' <- ps = [ virt cfg (to pos) VCurlyR | _ <- es ]
                                        ++ t
                                         : loop False ps' ts
 
-    | Sym CurlyR <- ty
-    , Explicit (Sym CurlyR) : ps' <- ps = [ virt cfg (to pos) VCurlyR | _ <- es ]
-                                       ++ t
-                                        : loop False ps' ts
-
     -- If we see the keyword `where`, we start a new virtual block
     | KW KW_where <- ty = t : virt cfg (to pos) VCurlyL
                             : loop True stack ts
@@ -270,6 +264,9 @@ layout cfg ts0
           punc | startBlock = []
                | otherwise  = [virt cfg (to pos) VSemi]
 
+          parensStack | startBlock = Virtual (col (from pos)) : stack
+                     | otherwise  =                            stack
+
           (es,ps) = span isVirtual stack
 
   -- We are the first token in a new block, push our column on the stack.