Log Message:
Update to 0.060

Upstream changes:
0.060           2018-06-16      T. R. Wyant
    \N{} now parses as the unknown token, not NoOp, regardless of the
    setting of 'use re qw< strict >;'. \N{} became unconditionally fatal
    in 5.28.0 (5.27.1, actually). The policy when the parse changes is
    to use the most-modern parse. Hence this change.

    As a side effect of this, the unknown token's explain() method now
    returns something -- normally the associated error.

    Add method remove_insignificant(). If the invocant isa Node, this
    returns a clone of the invocant with non-significant elements
    removed. Otherwise it returns either the invocant or nothing.