Support function in smart contract

[samuelmanzanera]

Is your feature request related to a problem?

Sometimes in smart contract we want to extract some part of the code to be reusable for multiple blocks or to structure more your code.
Since the AEIP17, we introduced a way to use function and to expose some through JSON rpc.

Describe the solution you'd like

To support this feature, we should adapt the interpreter parser to support function capability.

• Private function

We have to support this new syntax

fun myFun do
# Do something
end

Function can have 0 or N arity

They can only be called with the smart contract actions or condition blocks

• Pubilc function (exported)

We have to support this new syntax

export fun myFun do
  # Do something and return publically
end

Function can have 0 or N arity

They can be called by both private functions, actions or conditions blocks

Their scope should be limited: the parser should prevent :

• to mutate state
• use I/O functions
• contain loops to a certain amount of iterations or complexity

A default allowed complexity has to be defined to be consumed on each instruction (aka gas) limiting the execution of the external functions.

Additional context

No response

Epic

No response

[Neylix]

First implementations in #1181 #1199 #1202 #1212

Remaining tasks is complexity