New `using` method to ensure Closable resource is closed after executing a block of code

The name is inspired from C# `using` statement (to not confuse with `using` directive).
`using` works with any type with a `close()` method

No runtime cost for `Closable` type but structural type implicit conversion works with  any type with a `close()` method.
Replaced var by val in unit test
Merge pull request #15 from wajam/manu-closable-using

New `using` method to ensure Closable resource is closed after executing a block of code