Add str_slice method to StringNameSpace

[ayemjay]

Problem description

Extract out a substring using lazy api for example:

let df = df!(
    "s" => ["foobar", "barfoo"]
)?;

df
    .lazy()
    .with_columns([col("s").str().str_slice(0, Some(3))])
    .collect()?;

To give following result:

┌─────┐
│ s   │
│ --- │
│ str │
╞═════╡
│ foo │
│ bar │
└─────┘