* Updated to get latest aws-crt-ffi
* Added implementation for signing_result
* Added main thread queue and callback queueing to signing process
* Updated to get support for date on signing_config_aws
* added date support to SigningConfigAWS, fixed up reference issues on http message signing
* Broke C code into separate files, moved common stuff to php_aws_crt.h
* Use a unity build to make PHP do the right thing with our lib
* Updated aws-crt-ffi to v0.1.3 to get sigv4 changes
* Added CRT::isAvailable for safe runtime detection of CRT
* Fixed up test resource safety, added tests to prove it
* Added CI jobs for every version of PHP on linux (#26)
