Module structure

Hi @psxjoy @jipengfei-jpf!

I noticed that our current module structure is a bit confusing. It has:

* fastexcel: almost an alias of fastexcel-core?
* fastexcel-core: the core interfaces and implementations
* fastexcel-support: bundle spring-core's cglib
* fastexcel-test: testcode and examples

I'd suggest we have:

* (fastexcel or fastexcel-core, choose one of them): the core interfaces and implementations, merge fastexcel-test's testing code into here
* fastexcel-examples (and submodules): holding examples now in fastexcel-test, exclude all these modules when releasing

For fastexcel-support, we can leave it as is for now. But there should be someway to package it properly externally, like:

* https://github.com/apache/flink-shaded
* https://github.com/apache/ratis-thirdparty

The current solution has some issues like #332. I may look into the bundle things later, but the module structure things first.