Factor out a Pattern sum type from Expression (#685)

`Pattern` is intended to pilot some changes I would like to apply to all our sum types:
- The alternatives are expressed as derived classes rather than members of a `std::variant`.
- The alternatives are classes in the [style guide sense](https://google.github.io/styleguide/cppguide.html#Structs_vs._Classes), meaning they can have invariants, but can't have public data members.
- Creating an object is expressed using a constructor rather than a factory function.
- Accessing an alternative is expressed as a cast (using LLVM's RTTI system) rather than `std::get` or a `Get` method.

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

executable_semantics/syntax/BUILD

@@ -42,23 +42,7 @@ cc_library(
 
 cc_library(
     name = "paren_contents",
-    srcs = ["paren_contents.cpp"],
     hdrs = ["paren_contents.h"],
-    deps = ["//executable_semantics/ast:expression"],
-)
-
-cc_test(
-    name = "paren_contents_test",
-    srcs = ["paren_contents_test.cpp"],
-    env = {
-        # TODO(#580): Remove this when leaks are fixed.
-        "ASAN_OPTIONS": "detect_leaks=0",
-    },
-    deps = [
-        ":paren_contents",
-        "@llvm-project//llvm:gtest",
-        "@llvm-project//llvm:gtest_main",
-    ],
 )
 
 genrule(