Make the generated mapper available to the user

[octavz]

Hi,

There are situations where would be useful to build the mapper automatically, for example, if we need to dynamically add a where condition we could use query! or any other case where the fields won't change and then pass the Map structure to another function that would be able to modify the SQL inside.

Looking into the code that could either make it work by providing another SQL to the Map or by allowing "extracting" the mapper and building another Map with try_map.

Just being able to build and pass a "mapper" around would help and will minimize user code.

Not sure what are the implications but

pub struct Map<'q, DB: Database, F, A> {
    inner: Query<'q, DB, A>,
    mapper: F,
}

could change to

pub struct Map<'q, DB: Database, F, A> {
    pub inner: Query<'q, DB, A>,
    pub mapper: F,
}