Add str.startswith

[Uwe L. Korn]

• ✔️ pandas function
• ✔️ Python function
• ✔️ simple C++ STL function: https://stackoverflow.com/a/62280289/1689261
• ✔️ no need for a regular expression library
• ✔️ no need for a Unicode database
• ✔️ can pre-compute size, output is a boolean array

Pseudo-Code:

Inputs: pat

output = BooleanArray(len(rows))
for i, row in enumerate(rows):
    if isnull(row):
        output.setnull(i)
    else
        output[i] = (row[:len(pat)] == pat)