Add str.center method

[Uwe L. Korn]

• ✔️ pandas function
• ✔️ Python function
• ❌ C++ STL function
• ✔️ no need for a regular expression library
• ✔️ no need for a Unicode database
• ❌ cannot pre-compute output size without going over the data as some strings are wider then width (need no centering) and some aren't (need centering).

Pseudo-Code:

Inputs: width, fillchar

builder = StringBuilder()
for row in rows:
    if utf8_len(row) >= width:
        builder.append(row)
    else:
        n_missing = width - utf8_len(row)
        left = (n_missing // 2) + (n_missing % 2)
        right = (n_missing // 2)
        builder.add(fillchar * left + row + fillchar * right)