src/lexer-impl.lisp: Don't always skip a token.

src/lexer-impl.lisp: Don't always skip a token.

It's usually, or at least often, the wrong thing to do.  Consider, for
example, how to recover from trying to parse

	( list[x] )

when presented with

	( )

Instead, add a `:force-progress' argument to the `error' parser.  The
default is /not/ to consume a token; I think this is right.