Refine set syntax and more

The main changes are:

Removed the overloading of the equality operator to do set membership tests
and updated set generation syntax to align with the new membership test
syntax.

Also, got rid of the "[]" syntax in references and replaced with special "_"
character that is treated as a special iterator. Documented this behaviour in
the references section.

Also, fixed semantics so that document content is defined in terms of the rule
head which is in turn defined by instances of the body that evaluate to true.
E.g., instead of "p :- 7", we now write "p = 7", which is short for "p = 7 :-
true".

Author: tsandall

docs/CONCEPTS.md

@@ -86,11 +86,11 @@ Content-Type: text/plain
 
 package opa.examples
 
-violations[] = server :-
-	server = data.servers[]
-	server.protocols[] = "http"
-	server.ports[] = data.ports[i].id
-	data.ports[i].networks[] = data.networks[j].id
+violations[server] :-
+	server = data.servers[_]
+	server.protocols[_] = "http"
+	server.ports[_] = data.ports[i].id
+	data.ports[i].networks[_] = data.networks[j].id
 	data.networks[j].public = true
 ```
 
@@ -190,7 +190,7 @@ Conceptually, there are two kinds of documents in OPA:
 
 When defining policies, rules are written which contain expressions that reference documents. The language that rules are written in ("Opalog") lets you reference base documents and virtual documents in exactly the same way.
 
-<img src="https://cdn.rawgit.com/open-policy-agent/opa/9f5f1e6fa68fd0ee627122b9e5c8809519e5bba8/docs/data-model-logical.svg" />
+<img src="https://cdn.rawgit.com/open-policy-agent/opa/86672fab147c476cb8e8b0950b6c4fd48b5b2014/docs/data-model-logical.svg" />
 
 ## <a name="policies"></a> Policies
 
@@ -243,15 +243,15 @@ import data.servers                             # import the data.servers docume
 import data.networks                            # same but for data.networks
 import data.ports                               # same but for data.ports
 
-violations[] = server :-                        # a server exists in the violations set if:
-	server = servers[]                          # the server exists in the servers collection
-	server.protocols[] = "http"                 # and the server has http in its protocols collection
-	public_servers[] = server                   # and the server exists in the public_servers collection
+violations[server] :-                           # a server exists in the violations set if:
+	server = servers[_],                        # the server exists in the servers collection
+	server.protocols[_] = "http",               # and the server has http in its protocols collection
+	public_servers[server]                      # and the server exists in the public_servers set
 
-public_servers[] = server :-                    # a server exists in the public_servers set if:
-	server = servers[]                          # the server exists in the servers collection
-	server.ports[] = ports[i].id                # and the server is connected to a port in the ports collection
-	ports[i].networks[] = networks[j].id        # and the port is connected to a network in the networks collection
+public_servers[server] :-                       # a server exists in the public_servers set if:
+	server = servers[_],                        # the server exists in the servers collection
+	server.ports[_] = ports[i].id,              # and the server is connected to a port in the ports collection
+	ports[i].networks[_] = networks[j].id,      # and the port is connected to a network in the networks collection
 	networks[j].public = true                   # and the network is public
 ```