refactor: Flatten messagelayer (#732)

* refactor: Move messagelayer/message to tangle/

* refactor: Modify the method receiver and annotate the error messages

* refactor: Move messagelayer/payload to tangle/

* refactor: Modify the method receiver and annotate err messages in payload

* refactor: Move messagelayer/messagerequester to tangle/

* refactor: Modify the method receiver in requester

* refactor: Move messageparser/builtinfilter to tangle/

* refactor: Move messageparser/ to tangle/

* fix: Import fmt and minor fixes

* refactor: Move messagefactory/ to tangle/

* refactor: Move tipselector/ to tangle/

* refactor: Rename method receivers in tipselector

* refactor: Move messagelayer/tangle to tangle/

* refactor: Rename method receivers and annotate error messages in tangle

* refactor: Move messagelayer/test to tangle/

* fix: Update to the develop branch

* refactor: Create data payload in testing message creation

* fix: Fix 



* fix: Rename for consistency

* fix: Move message related const from payload.go to message.go

* refactor: Use single character for method receivers

* fix: Fix the unit test by using the correct variable name

* Feat: merged marshal changes

* Feat: removed superfluous test

* Fix: fixed wrong comment

* refactor: Annotate error messages in marshal functions

* refactor: Import fmt package

* refactor: Aggregate all filters into filter.go

* refactor: Rename Factory to MessageFactory to clarify the functionality

* refactor: Rename Parser to MessageParser to clarify the functionality

* refactor: Rename Requester to MessageRequester to clarify the functionality

* refactor: Rename Parse to FromMarshalUtil for naming consistency

* refactor: Fix comments

Co-authored-by: Hans Moog <hm@mkjc.net>