4.5.0: coeffects support, permit strict executables

This release:

• adds addCoeffect() and addCoeffects() to functions and methods
• permits strict-mode files with shebangs (e.g. #!/usr/bin/env bash)
• pseudomain functions remain banned in strict files; instead, use <<_Entrypoint>> functions in strict mode executables
• requires Hack/HHVM 4.93 or above