[lang][grammar] Improve type declaration. (variables vs action return)

[srodriguez]

A variable is declared as

var String s = "xx"

while an action return type is declared with:

def myString : String {
 "xx"
}

This should be unified.

[gallandarakhneorg]

I prefer the "name : type = value" syntax because it is close to the UML syntax.

[srodriguez]

Following our exchange, we will use a scala inspired declaration:

var name : String = "my agent"
val name : String = "my agent"

def something(string : String) : String {
//...
}

[gallandarakhneorg]

Approved.

[ngaud]

Approved.