cocoa_bridge: Continue outline of Objective C callable

Add some more structure to the function that will be called from
Objective C code.

* Give it a name, `parse_mappings` (not very thoroughly considered)
* Initialise some Foundation data structures to mirror our Rust ones
* Add the beginning of a struct that will be the ObjC version of our
  `MapGroup` struct, containing `NSDictionary`ies.
* Move the `extern crate` into `lib.rs`, I keep forgetting that's where
  they go.
* Add a test that just calls `parse_mappings` to check that the code
  compiles.

parser::MapGroup:
Make both its fields public to enable us to access them from
`cocoa_bridge`.

Used this helpful example as a reference for the Rust Cocoa code:
https://github.com/servo/core-foundation-rs/blob/c99c05c/cocoa/examples/hello_world.rs

1 files changed, 2 insertions, 0 deletions

diff --git a/src/lib.rs b/src/lib.rs
index c2d2f9f..e6260ed 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,3 +1,5 @@
+extern crate cocoa;
+
 #[macro_use]
 extern crate combine;