Allow namespace override.

This change allows a

 #[namespace (namespace = A::B)]

attribute for each item in a cxx::bridge.

We now have a fair number of different types of name floating
around:
* C++ identifiers
* C++ fully-qualified names
* Rust identifiers
* Rust fully-qualified names (future, when we support sub-modules)
* Items with both a Rust and C++ name (a 'Pair')
* Types with only a known Rust name, which can be resolved to a
  C++ name.

This change attempts to put some sensible names for all these
things in syntax/mod.rs, and so that would be a good place to start
review.

At the moment, the Namespace (included in each CppName)
is ruthlessly cloned all over the place. As a given namespace is
likely to be applicable to many types and functions, it may
save significant memory in future to use Rc<> here. But let's not
optimise too early.
diff --git a/syntax/atom.rs b/syntax/atom.rs
index 6e5fa88..7d0ef6b 100644
--- a/syntax/atom.rs
+++ b/syntax/atom.rs
@@ -81,7 +81,7 @@
 impl PartialEq<Atom> for Type {
     fn eq(&self, atom: &Atom) -> bool {
         match self {
-            Type::Ident(ident) => ident == atom,
+            Type::Ident(ident) => ident.rust == atom,
             _ => false,
         }
     }