Header-only + lib rework

[skypjack]

@stefanofiorentino I created this issue to keep track of changes. I'm pushing on experimental and I'd kindly invite you to join the project as a developer if you like.

This is a brief, yet incomplete TODO list:

• ~Split util.hpp into util.h and util.cpp (eg Utilities).~
• ~Split emitter.hpp into emitter.h and emitter.cpp (eg ErrorEvent).~
• ~Make it export the right targets according with the user configuration (either a target for the header-only library or two targets for the static/shared lib).~
• ~Some functions from fs.h can be moved to fs.cpp (eg scandirNext and readdirSync).~
• ~Many constructors and some other functions here and there should be moved to the cpp files.~
• ~Clean up include directives so as to reduce dependencies on header files.~
• ...

[skypjack]

I'm closing this issue since it seems we did everything. :+1: Good job and thank you for your help @stefanofiorentino :heart: