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.