Berry class int64 made immutable

[s-hadinger]

Description:

Change Berry int64 class to make instances immutable, which avoids hard to find bugs, in exchange of a slightly less efficient implementation. We take into account that use of int64 is rare.

Checklist:

• The pull request is done against the latest development branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works with Tasmota core ESP8266 V.2.7.6
• The code change is tested and works with Tasmota core ESP32 V.2.0.14
• I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass