Given a list of integers sorted in ascending order (with possible duplicates), return the index of the first occurrence of target, or -1 if it's not there.
Your solution must run in time, so no scanning the list. Don't use the bisect module, either. Write the search yourself.
Example
first_occurrence([1, 2, 2, 2, 3, 5], 2) # 1
first_occurrence([1, 2, 3], 4) # -1