Refactor the appender to use a dedicated vector-type

[taniabogatsch]

This PR moves all previous colInfo functions from the appender.go file into a dedicated vector.go file. Operating directly on these vectors makes the code more concise and will allow me to add future capabilities neatly. Currently planned are the following improvements to the Appender.

• implicit type casting (e.g., after unmarshalling a JSON file)
  • add a function func (vec *vector) prepareRow(value driver.Value, tryCast bool) error improving type checks and adding implicit casting capabilities.
• better nil/NULL handling (e.g., see discussion here: Initialise column types during appender creation #171)