* Moved not required argument logic into function
Seemed more logical, as you can't have a required argument after the
not required argument. Usage:
```rs
FunctionBuilder::new("skeleton_version", skeleton_version)
.arg(Arg::new("req_arg1", DataType::String))
.arg(Arg::new("req_arg1", DataType::String))
.not_required()
.arg(Arg::new("not_req_arg1", DataType::String))
.build()
```
* Added zend_value implementation
Helper functions to retrieve the value from zvals
* Started work on argument parser
* Added PHP version support notice
Due to the way the Zend API changes and the fact that the
stable interface is exposed via C macros, there is no
support for versions lower than PHP 8.0. In the future
we can support PHP 7.4 through Rust features.
* Added TryFrom implementations for Zval -> Types
* Added ability to select arguments from ExecutionData
* Added ability to parse arguments and retrieve val
See `example/skel/src/lib.rs` for usage.
