SC: add reduce/3 action

[bchamagne]

Description

Implement a reduce/3 action in the smart contracts.
With the reduce alone we can do all the list manipulation we need.
We could add more convenience functions later such as map/filter.

Syntax:

list = [1,2,3]
sum = reduce(list, 0, fn item, acc ->  
    item + acc
end)

Fixes #826

Current limitation to discuss

• Impossible to call actions from within the reduce (set_content/add_uco_transfer etc.)
• Impossible to have a reduce in a reduce
• Is the elixir lambda syntax good?

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Unit tests added to the suite

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[bchamagne]

Assignations (=) do not work in reduce. I'll fix this ASAP.

[bchamagne]

dot access (x.y) does not work on lambda arguments

        content = reduce(list, [], fn tx, acc ->
            append(acc, tx.address)
        end)

[bchamagne]

I will create a new branch and a new PR.