Varchar type

[juanjux]

Part of the fix for: src-d/gitbase/issues/824 (the missing part is using it on the columns / types of gitbase where superset expects varchars).

This adds VarChar as an specific type. As happens with Array, it's a parametrized type that will be returned by a function with a specific length. It is convertible from and to Text and string and will return true for IsText so it should work for all functions using Text.

There is a "TO-DO" comment I added in the NewLiteral function, any suggestion is welcomed:

	// TODO(juanjux): we should probably check here if the type is sql.VarChar and the
	// Capacity of the Type and the length of the value, but this can't return an error