Split cfengine_stdlib.cf into pieces for 3.5.1 and higher and provide fallback for 3.5.0 and older

* provide `libraries/cfengine_stdlib.cf` as fallback for 3.5.0 and older versions
* starting with 3.5.1, put all the pieces of the stdlib in `MAJOR.MINOR/*.cf` split by promise type
* we will remove `libraries/cfengine_stdlib.cf` for CFEngine 3.7, at which point the directories will be `3.5/*.cf`, `3.6/*.cf`, and `3.7/*.cf`.  Until then we will keep it so clients upgrading from 3.5.0 or older don't fail.