Introduce a ContextBuilder struct (#3698)

The way to create a Context is now rather burdensome, users have to
create and import a bunch of things just to get a Context.  So let's
introduce a builder.

Notice that the builder can only produce an open context, if the
context can not be opened it is dropped.  This is on purpose, the
Context itself can become RAII again at some point by doing this.
Only the FFI needs to have the concept of an open and a closed
Context.

CHANGELOG.md

@@ -31,6 +31,8 @@
 
 
 
+- Rust: Introduce a ContextBuilder #3698
+
 ### Changes
 - allow sender timestamp to be in the future, but not too much