Add str.cat

[Uwe L. Korn]

• ✔️ pandas function
• ✔️ Python function: +
• ✔️ C++ STL function: +
• ✔️ no need for a regular expression library
• ✔️ no need for a Unicode database as we just append
• ✔️/ ❌ can pre-compute output depending on the parameters

Pseudo-Code:

Inputs: others, sep, na_rep

# TODO: Write pseudo code for all of the possible variants