Re-organize types module and remove logic from `sys` module

[kjvalencik]

The sys module was moved from a neon-runtime crate when the C++/NAN backend was removed. Originally it made sense to have logic there in order to abstract over the backend. However, now that there is a single backend it's no longer necessary to restrict calls to Node-API to this module. In fact, the extractor PR makes heavy use of sys functions for performance.

A cleaner and easier to follow model is a module for each JavaScript type.

neon
└── types_impl
    ├── buffer
    │   └── mod.rs
    ├── number
    │   └── mod.rs
    └── string
        └── mod.rs

In this structure, we would move TryFromJs implementations. In many cases, we could de-duplicate code by making fn value(..) implementations delegate to try_from_js. For example, JsString::value:

impl JsString {
    pub fn value<'cx, C>(&self, cx: &mut C) -> String
    where
        C: Context<'cx>,
    {
        String::try_from_js(self.upcast())?.unwrap()
    }
}