Remove dc_open call

A new context is now created by calling Context::new and therefore you
always have a valid context.  This is much more in Rust style and will
allow a lot of furture simplifications on the context itself.

The FFI layer has not yet been adjusted in this commit and thus will
fail.

examples/repl/main.rs

@@ -15,8 +15,8 @@ extern crate rusqlite;
 
 use std::borrow::Cow::{self, Borrowed, Owned};
 use std::io::{self, Write};
+use std::path::Path;
 use std::process::Command;
-use std::ptr;
 use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::{Arc, Mutex, RwLock};
 
@@ -44,7 +44,7 @@ use self::cmdline::*;
 
 // Event Handler
 
-unsafe extern "C" fn receive_event(
+fn receive_event(
     _context: &Context,
     event: Event,
     data1: uintptr_t,
@@ -387,15 +387,15 @@ impl Highlighter for DcHelper {
 impl Helper for DcHelper {}
 
 fn main_0(args: Vec<String>) -> Result<(), failure::Error> {
-    let mut context = dc_context_new(Some(receive_event), ptr::null_mut(), Some("CLI".into()));
-
-    if args.len() == 2 {
-        if unsafe { !dc_open(&mut context, &args[1], None) } {
-            println!("Error: Cannot open {}.", args[0],);
-        }
-    } else if args.len() != 1 {
+    if args.len() < 2 {
         println!("Error: Bad arguments, expected [db-name].");
+        return Err(format_err!("No db-name specified"));
     }
+    let context = Context::new(
+        Box::new(receive_event),
+        "CLI".into(),
+        Path::new(&args[1]).to_path_buf(),
+    )?;
 
     println!("Delta Chat Core is awaiting your commands.");