Update the Package format to the SPM 4 format, made the library .dynamic

[fadi-botros]

Please don't put it on the master branch (for master, please use

.library(name: "PerfectLib", targets: ["PerfectLib"])

instead of

.library(name: "PerfectLib", type: .dynamic, targets: ["PerfectLib"])

Until Apple fully supports dynamic libraries in Swift Package Manager, (As a temporary solution, try my fork of Swift Package Manager (not tested on Linux) https://github.com/fadi-botros/swift-package-manager)

[CLAassistant]

All committers have signed the CLA.