Allow PHP 8.4 to be used (#25)

Also, include it in the test matrix. At the same time, time to let go of
old versions.

Reduce log level for events that make most of the log volume with lea…

…st of newsworthiness (#22)

These messages make up most of the log volume, yet only indicate that
nothing has happened.

Let's reduce them to the `debug` log level.

Update dependencies (#21)

Allow the usage of PHP 8.3 (#17)

Add type hints for static linting with Psalm (#10)

This PR adds type hints and docblocks to support static analysis of your custom mapper classes with Psalm.

Most notably, `SourceAdapter`, `DestinationAdapter` and `Mapper` implementations can make use of templated types in Psalm, which is a great help due to the generic (`object` based) approach in this package.

The types are:

* `Ts` type returned by the `SourceAdapter`
* `Tr` type returned by `DestinationAdapter::getObjectsOrderedById()`, `r` for "reading"
* `Tw` type used by the `DestinationAdapter` when writing back to the destination system.

Introduce log message for unmappable objects

Add a new "unmappable" result type

This can be used if a source object should be treated as if it does
not exists (don't insert/delete when present). In some cases, this
cannot be detected unless the mapping is attempted.

Normally, such objects should not be returned from the SourceAdapter
in the first place.

Bump requirements to PHP 7.1

Fix wrong version number in CHANGELOG

Merge pull request #3 from webfactory/add-constructor-helpers

Add more readable constructor methods