Add .Flow v2 string operations

[nicolasbotanic]

Please check https://github.com/SeedVault/flow#string-functions. You will see all .Flow v2 instructions needed to gain string operations.

**Do not include $string instruction. This is a reserved instruction with no current support **

You can see the code of others simple .Flow instructions in file engines/dotflow2/core_functions.py methods df2_eq, df2_gt, df2_lt, etc... Add the code in this file.

You can easily test .Flow v2 instructions using the provided console:

BBOT_ENV=development python -m channels.console.app joe testbot 1 nodebug

This will enter to the provided demo bot's console as userid "joe"
You can run any .Flow instruction with the command df2:

Expected result with $upper:

You: :df2 upper('Hello!')
Testbot: HELLO!

And so on...

It is welcomed and encouraged to add more instructions than what we have in the documentation