Add methods and visibility

[fichtelmann]

I've added the implemented methods of a struct to the PUML output.
The visibility of fields and methods is also respected.

The output of my test file:

pub struct MyStruct {
    pub public_field: i32,
    private_field: i32,
    second_struct: SecondStruct,
}

impl MyStruct {
    pub fn new(public_field: i32, private_field: i32, second_struct: SecondStruct) -> Self {
        MyStruct {
            public_field,
            private_field,
            second_struct
        }
    }

    pub fn get_private_field(&self) -> i32 {
        self.private_field
    }

    fn increment_private_field(&mut self) {
        self.private_field += 1;
    }
}

pub struct SecondStruct {
    pub second_public_field: i32,
    second_private_field: i32,
}

impl SecondStruct {
    pub fn get_second_private_field(&self) -> i32 {
        self.second_private_field
    }
}
```

results in this PUML output now:

```puml
@startuml

class "MyStruct" {
    + public_field: i32
    - private_field: i32
    - second_struct: SecondStruct
    + new(public_field: i32, private_field: i32, second_struct: SecondStruct)
    + get_private_field()
    - increment_private_field()
}

class "SecondStruct" {
    + second_public_field: i32
    - second_private_field: i32
    + get_second_private_field()
}
"MyStruct" <-- "SecondStruct"



@enduml
```