That tells a vector explicitly to parse until it hits the end of the
input, without having to use the look-ahead machinery to figure out when
to stop.

If a field is marked with ``&try``, a later call to ``self.backtrack()``
anywhere down in the parse tree will return to that position and
continue there. (This is ported over the old prototype).

Example:

    export type test = unit {
        a: bytes &length=4;
        foo: Foo &try;
        bar: Bar;
        b: bytes &length=6;

        on %done { print self; }
    };

    type Foo = unit {
        a: int8 { print "Foo.a", self; }
        b: int8 { print "Backtracking"; self.backtrack(); }
        c: int8 { print "Foo.c", self; }
    };

    type Bar = unit {
        a: int8 { print "Bar.a", self; }
        b: int8 { print "Bar.b", self; }
        c: int8 { print "Bar.c", self; }
    };

    # printf '1234\001\002\003567890' | pac-driver backtrack.pac2
    Foo.a <a=1, b=(not set), c=(not set)>
    Backtracking
    Bar.a <a=1, b=(not set), c=(not set)>
    Bar.b <a=1, b=2, c=(not set)>
    Bar.c <a=1, b=2, c=3>
    <a=1234, foo=(Null), bar=<a=1, b=2, c=3>, b=567890>

Closes #25.

When an example contained multiple commands separated by semicolons,
we'd execute them separately but still include all of them into each of
the command lines emitted into the docs.

(Couple of doc outputs show slightly changed content, including updates
into this commit.)