[Proposal] improvements on the Iterator Function

Some times the Iterator function required early break, and in this situation the developer probably want to yield a value and early break the function. Then ``Yield`` and ``Return`` will be used. And I think that separate the ``Yield`` and ``Return`` keyword into two statement line probably looks terrible:

```vbnet
Public Iterator Function Blablabla(boo As Boolean) As IEnumerable(Of Integer)

If (boo) Then
' Yield value and early break this function
Yield 1
Return
End If

If blablablabls.... Then
' Just early break this function
Return
End If

' blabla
' Just yield value
Yield AnotherValue
End Function
```

Probably merge the ``Yield`` and ``Return`` into one statement can makes the vb code looks more elegant:

```vbnet
Public Iterator Function Blablabla(boo As Boolean) As IEnumerable(Of Integer)

If (boo) Then
' Yield value and early break this function
Yield Return 1
End If

If blablablabls.... Then
' Just early break this function
Return
End If

' blabla
' Just yield value
Yield AnotherValue
End Function
```