Increased consistency, support for vec/dict/keyset, support for HHVM 3.23

• adds support for the upcoming HHVM 3.23 release
• in general, methods that previously took collections will now take Traversable or KeyedTraversable instead, allowing them to be used with either Hack collections or Hack arrays
• methods that returned collections now return Hack arrays
• added support for running hackfmt-formatted output: add Facebook\HackCodegen\HackfmtFormatter as the formatter in your configuration object
• <?hh // strict is now the default for generated files
• added CLI script to verify signatures - this should be available as vendor/bin/hh-codegen-verify-signatures in projects that depend on hack-codegen
• renamed addVar() (add a member variable) to addProperty(), and added addProperties()
• replaced setGenericsDecl() with addGenerics() and addGeneric(), which take a string describing a generic
• HackCodegenConfig no longer takes parameters - instead, call ->withRootDir($root) on the object to specify the root directory
• consistency improvements:
  • addCase() now takes a value renderer as well as a value
  • endSwitch_() was renamed to endSwitch()
  • replaced setserAttribute() with addUserAttribute(), which takes a value renderer as well as a value, and addEmptyUserAttribute()
  • renamed setParameters() to addParameters()
  • split addComment() to addComment() and addCommentf()
  • split addIf() to addIf() and addIff()
  • split addLine() to addLine() and addLinef()