#65 Add startsWith and endsWith JAPE operators

[Phil Gooch]

startsWith, e.g.

{X startsWith {Y.majorType == prefix} }

(matches if X.length >= Y.length and the relative offset of Y within X is 0)

endsWith e.g.

{X endsWith {Y.majorType == suffix} }

(matches if X.length >= Y.length and the relative offset of Y within X is X.length - Y.length )