Scope assignation returns assigned value

[Neylix]

Description

Fixes an issue in smart contract language where it could be possible for a user to have an atom in a variable.
When assigning a value to a variable, we call Scope.write_cascade, this function was returning :ok. The problem was that we could create a function where the last statement is a assignation, and so the function returns :ok

action ... do
  my_var = bad_return()
  log(my_var) 
  # :ok
end

fun bad_return() do
  # Assignation returns :ok
  var = "toto"
end

The fix is to have the same behavior than elixir, the assignation return the assigned value

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Create a simple contract with a function that assign a value in it's last statement

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