[Proposal] "In" keyword relationship operator

Generally, the word ``In`` make sense for these situations:

+ Dictionary has the Key?
+ Collection contains the element?
+ Number in a specific range?
+ File exists in a directory?
+ Resource exists in a specific source?

So that by introduce the existed ``In`` keyword As a new operator, then we can makes the vb code more simple and human natural Language:

```vbnet
Public Class TableType

Dim table As New Dictionary(Of String, Integer)

Public Shared Operator In(key$, table As TableType) As Boolean
Return table.table.ContainsKey(key)
End Operator

End Class

Dim c As New TableType
Dim x$

If x In c Then
End If
```

```vbnet
Public Class CollectionType

Dim list As New List(Of Integer)

Public Shared Operator In(x%, list As CollectionType) As Boolean
Return list.list.IndexOf(x) > -1
End Operator

End Class

Dim c As New CollectionType
Dim x%

If x In c Then
End If
```

```vbnet
Public Class IntRange
Dim range%()

Public Shared Widenning Operator CType(l%()) As IntRange
Return New IntRange With {
.range = l
}
End Operator

Public Shared Operator In(x%, r As IntRange) As Boolean
Return Array.IndexOf(r.Range, x) > -1
End Operator
End Class

' If [1: 10] means integer range 1 to 10, then we can
Dim x%

If x In [1:10] Then
End If
```

And combine this new ``In`` keyword with the Linux bash file search API syntax in VisualBasic language:

https://github.com/xieguigang/GCModeller/blob/master/src/GCModeller/CLI_tools/eggHTS/CLI/2.%20DEP.vb#L196

to makes the VB style in more Natural Language

```vbnet
Public Class Directory
Dim DIR$

Public Shared Widening Operator CType(DIR$) As Directory
Return New Directory With {
.DIR =DIR
}
End Operator

Public Shared Operator In(file$, DIR As Directory) As Boolean
Return (ls -r <= DIR).IndexOf(file.BaseName) > -1
End Operator
End Class

' In another style
Public Class Directory
Dim files As CollectionType(Of String)

Public Shared Widening Operator CType(DIR$) As Directory
Return New Directory With {
.files = (ls -l -r -"*.*" <= DIR)
}
End Operator

Public Shared Operator In(file$, DIR As Directory) As Boolean
Return file$ In DIR.files
End Operator
End Class

Dim file$
Dim DIR As Directory = "C:\test"

If file In DIR Then
End If

If file$ In (ls -l -r -"*.xml" <= "D:\data\") Then
End If

' or even more simple
Public Class File
Dim file$

Public Shared Operator In (file As File, DIR$) As Boolean
With file
Return (ls -l -r - .File.ExtensionName <= DIR).IndexOf(.File) > -1
End With
End Operator
End Class

Dim file As File

If file In "C:\test\" Then
End If
```

Compared with the ``List.IndexOf``, ``Dictionary.ContainsKey``, In keyword operator just using 2 character in our code, simple and make sense.