pin Connection

[ghedo]

Due to the fact that we store a pointer to the Connection object inside
its own Handshake context, Connection objects cannot be moved (which is
why we return a boxed object from its constructor). Using Pin on it
makes the compiler enforce this.

Fixes #45.

---

Note this requires Rust 1.39.

[LPardue]

Nightly builds are failing due to errors like

error: `extern` fn uses type `Config`, which is not FFI-safe

   --> src/ffi.rs:81:50

    |

81  | pub extern fn quiche_config_new(version: u32) -> *mut Config {

    |                                                  ^^^^^^^^^^^ not FFI-safe

    |

    = note: `-D improper-ctypes` implied by `-D warnings`

    = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct

    = note: this struct has unspecified layout

note: type defined here

   --> src/lib.rs:372:1

[ghedo]

Looks like it's just the newer rust nightly, as master also fails :/

[ghedo]

#251 should "fix" it.

[ghedo]

Added some more context to the commit message.