Support for anonymous function

[danilhendrasr]

Yalija's function declaration syntax performs two independent operations. It creates a function and also binds it to a name. This improves usability for the common case where we do want to associate a name with the function. But in functional-styled code, we often want to create a function to immediately pass it to some other function or return it. In that case, it doesn’t need a name.

Languages that encourage a functional style usually support anonymous functions or lambdas—an expression syntax that creates a function without binding it to a name. Maybe yalija should implement it too.