[whisk]

Problem

InsertAt can corrupt bits in the first word when inserting to positions in word 2 or beyond. That was very difficult to catch even with massive fuzzing.

b2 := New(0).Set(9) // 1st word
b2.InsertAt(64)     // 2nd word
b2.Test(9) // FALSE, expected to be TRUE

Fix

A good simplification of InsertAt was the use of extendSet and covering an edge case when idx > length.