Add str.slice

[Uwe L. Korn]

• ✔️ pandas function
• ✔️ Python function: […]
• ✔️/❌ simple C++ STL function: http://www.cplusplus.com/reference/string/string/substr/ (no UTF-8 support)
• ✔️ no need for a regular expression library
• ✔️ no need for a Unicode database
• ❌ cannot pre-compute size, output depends on the occurence of UTF-8 characters

Pseudo-Code:

Inputs: start, end, step

builder = StringBuilder()
for row in enumerate(rows):
    if isnull(row):
        builder.addnull()
    else
        builder.add(utf8_slice(row, start, end, step))