Addr Keyword Implementation (#1255)

* Implement addr keyword

* Add files that were deleted during merge

Some files got deleted during merging trunk because of the executable
semantics rename.

* Implement changes from code review.

Major changes:
- Rename AddrBindingPattern to AddrPattern
- Fix AddrPattern related changes in fuzzing

* Update documentation for GetField

* Fix AddPattern according to @zygoloid's suggestions

* Apply @zygoloid's changes to method comment

* Add a multi-component field example

* Incorporate AST changes to fuzzer corpus

* Refactor the deduced_param_list grammar rule

Suggested by @zygoloid

explorer/ast/pattern.cpp

@@ -56,6 +56,9 @@ void Pattern::Print(llvm::raw_ostream& out) const {
     case PatternKind::VarPattern:
       out << "var" << cast<VarPattern>(*this).pattern();
       break;
+    case PatternKind::AddrPattern:
+      out << "addr" << cast<AddrPattern>(*this).binding();
+      break;
   }
 }
 
@@ -87,6 +90,9 @@ void Pattern::PrintID(llvm::raw_ostream& out) const {
     case PatternKind::VarPattern:
       out << "var ...";
       break;
+    case PatternKind::AddrPattern:
+      out << "addr ...";
+      break;
     case PatternKind::ExpressionPattern:
       out << "...";
       break;
@@ -112,6 +118,8 @@ auto VisitNestedPatterns(const Pattern& pattern,
                                  visitor);
     case PatternKind::VarPattern:
       return VisitNestedPatterns(cast<VarPattern>(pattern).pattern(), visitor);
+    case PatternKind::AddrPattern:
+      return VisitNestedPatterns(cast<AddrPattern>(pattern).binding(), visitor);
     case PatternKind::BindingPattern:
     case PatternKind::AutoPattern:
     case PatternKind::ExpressionPattern: