Opaque declarations, and measure and reset operations.

The instructions of QASM are classified in a hierarchy, I've tried to
implement them in the form of several enums but extending enums is not
supported in Rust (yet).

https://stackoverflow.com/questions/25214064/can-i-extend-an-enum-with-additional-values

The hierarchy is mostly related to QuantumOperation and UnitaryOperation.
A UnitaryOperation is always a QuantumOperation but not the other way around.

I went for the wrapping alternative which will make getting the value a bit
tedious. I hope to be able of implementing some traits to ease this task but
without rust-lang/rfcs#1450 it could be difficult.

In reality, this hierarchy is not strictly neccessary, I could have
grouped all the instructions in the same enum and let the grammar accept
those programas that use the different operations correctly, but I wanted
to experiment with a proper separation of instruction types. Let's see.

Perhaps, a different approach would be following this Thin Traits proposal:
http://smallcultfollowing.com/babysteps/blog/2015/10/08/virtual-structs-part-4-extended-enums-and-thin-traits/

src/grammar/open_qasm2/ast.rs

@@ -9,7 +9,9 @@ pub struct OpenQasmProgram {
 pub enum Statement {
   QRegDecl(String, usize),
   CRegDecl(String, usize),
-  GateDecl(String, Vec<String>,  Vec<String>, Vec<GateOperation>)
+  GateDecl(String, Vec<String>,  Vec<String>, Vec<GateOperation>),
+  OpaqueGateDecl(String, Vec<String>, Vec<String>),
+  QuantumOperation(QuantumOperation)
 }
 
 #[derive(Debug, PartialEq)]
@@ -18,13 +20,18 @@ pub enum GateOperation {
   Barrier(Vec<String>)
 }
 
+#[derive(Debug, PartialEq)]
+pub enum QuantumOperation {
+  Unitary(UnitaryOperation),
+  Measure(Argument, Argument),
+  Reset(Argument)
+}
+
 #[derive(Debug, PartialEq)]
 pub enum UnitaryOperation {
   U(Expression, Expression, Expression, Argument),
   CX(Argument, Argument),
-  GateExpansion(String, Vec<Expression>, Vec<Argument>),
-  Measure(Argument, Argument),
-  Reset(Argument)
+  GateExpansion(String, Vec<Expression>, Vec<Argument>)
 }
 
 #[derive(Debug, PartialEq)]

src/grammar/open_qasm2/open_qasm2.lalrpop

@@ -17,20 +17,30 @@ pub Program: Vec<ast::Statement> = {
 
 pub Statement: ast::Statement = {
   RegisterDeclaration,
-  <GateDeclaration> "}" => ast::Statement::GateDecl(<>.0, <>.1, <>.2, vec![]),
-  <decl:GateDeclaration> <ops:GateOperationList> "}" => {
-    ast::Statement::GateDecl(decl.0, decl.1, decl.2, ops) }
+  <GateDeclaration> "{" "}" => ast::Statement::GateDecl(<>.0, <>.1, <>.2, vec![]),
+  <decl:GateDeclaration> "{" <ops:GateOperationList> "}" => {
+    ast::Statement::GateDecl(decl.0, decl.1, decl.2, ops) },
+  "opaque" <GateDeclaration> ";" =>
+    ast::Statement::OpaqueGateDecl(<>.0, <>.1, <>.2),
+  QuantumOperation => ast::Statement::QuantumOperation(<>)
 };
 
 RegisterDeclaration: ast::Statement = {
   "qreg" <Id> "[" <Nninteger> "]" ";" => ast::Statement::QRegDecl(<>),
   "creg" <Id> "[" <Nninteger> "]" ";" => ast::Statement::CRegDecl(<>)
 };
 
+QuantumOperation: ast::QuantumOperation = {
+  "measure" <Argument> "->" <Argument> ";" =>
+    ast::QuantumOperation::Measure(<>),
+  "reset" <Argument> ";" => ast::QuantumOperation::Reset(<>),
+  UnitaryOperation => ast::QuantumOperation::Unitary(<>)
+};
+
 GateDeclaration: (String, Vec<String>,  Vec<String>) = {
-  "gate" <id:Id> <args:IdList> "{" => (id, vec![], args),
-  "gate" <id:Id> "(" ")" <args:IdList> "{" => (id, vec![], args),
-  "gate" <Id> "(" <IdList> ")" <IdList> "{" => (<>)
+  "gate" <id:Id> <args:IdList> => (id, vec![], args),
+  "gate" <id:Id> "(" ")" <args:IdList> => (id, vec![], args),
+  "gate" <Id> "(" <IdList> ")" <IdList> => (<>)
 };
 
 GateOperationList: Vec<ast::GateOperation> = {