Implement "scanset" semantics for manipulator skip, in place of exact-match.
skip
That is, sin | skip("abc") now consumes all input before the "+" on input "cccccb+".
sin | skip("abc")
Exact-match semantics is available with just sin | "abc".
sin | "abc"
(Prior exact-match semantics was discovered by surprise, while debugging a test program for an upcoming commit.)