Feature: Object support

[FxMorin]

This PR adds primitive support for maps using objects in either the keys or values.
The performance benefits are still clearly there, while providing less memory usage.

Other changes:

• The readme has been modified to show performance benchmark results for object keys.
• Added ConcurrentMap comparison in JMH benchmark. Since its also a viable alternative (when not considering memory).
• Cleaned up the builder code to reduce DRY

This PR drastically changes the structure of the code, and would require a major version change.

[magicprinc]

@FxMorin Please, see also optimized “slim” version: https://github.com/magicprinc/fastutil-concurrent-wrapper

less code and RAM (fewer wrappers)

Branch "dev" has ultimate "zero fat" version.

PS: for long→object Maps you could find jctools' org.jctools.maps.NonBlockingHashMapLong interesting too.

[magicprinc]

@FxMorin Please see https://github.com/magicprinc/fastutil-concurrent-wrapper/blob/master/src/main/java/com/trivago/fastutilconcurrentwrapper/longkey/StripedNonBlockingHashMapLong.java

I would be very happy if you write more tests for this class 🙏

Benchmark StripedNonBlockingHashMapLong vs ConcurrentLongObjectMap is also very welcomed

As you can see, jars are available at
https://jitpack.io/#magicprinc/fastutil-concurrent-wrapper

[magicprinc]

@FxMorin If you see other classes: they are extremely compact now (several times shorter than originals). So it's very easy to add every other missing key-value combination.