Add columnName (sqlite3_column_name)

This adds support for `sqlite3_column_name`, which returns the name of a result column.  Example:

```
> import Database.SQLite3
> conn <- open ":memory:"
> exec conn "CREATE TABLE foo (abc TEXT, d INT)"
> stmt <- prepare conn "SELECT *, randomblob(32) FROM foo"
> columnName stmt 0
Just "abc"
> columnName stmt 1
Just "d"
> columnName stmt 2
Just "randomblob(32)"
> columnName stmt 3
Nothing
```

@nurpax: This might be an easy way to provide column information for conversion errors in sqlite-simple (https://github.com/nurpax/sqlite-simple/issues/20).