Add \exhaustive\ annotation to exhaustive match blocks

Ponyc recently added an \exhaustive\ annotation for match expressions.
When present, the compiler will fail compilation if the match is not
exhaustive. This protects against future breakage if new variants are
added to a union type.

Author: SeanTAllen

appdirs/_test.pony

@@ -54,7 +54,7 @@ primitive \nodoc\ _AppDirsTestUtil
     h: TestHelper,
     loc: SourceLoc = __loc) ?
   =>
-    match expected.home_dir
+    match \exhaustive\ expected.home_dir
     | _ExpectError =>
       h.assert_error(
         {()? => app_dirs.user_home_dir()? }
@@ -65,7 +65,7 @@ primitive \nodoc\ _AppDirsTestUtil
         app_dirs.user_home_dir()?
         where loc = loc)
     end
-    match expected.user_data_dir
+    match \exhaustive\ expected.user_data_dir
     | _ExpectError =>
       h.assert_error(
         {()? => app_dirs.user_data_dir()? }
@@ -76,7 +76,7 @@ primitive \nodoc\ _AppDirsTestUtil
         app_dirs.user_data_dir()?
         where loc = loc)
     end
-    match expected.site_data_dirs
+    match \exhaustive\ expected.site_data_dirs
     | _ExpectError =>
       h.assert_error(
         {()? => app_dirs.site_data_dirs()? }
@@ -88,7 +88,7 @@ primitive \nodoc\ _AppDirsTestUtil
         where loc = loc)
     end
 
-    match expected.user_config_dir
+    match \exhaustive\ expected.user_config_dir
     | _ExpectError =>
       h.assert_error(
         {()? => app_dirs.user_config_dir()? }
@@ -99,7 +99,7 @@ primitive \nodoc\ _AppDirsTestUtil
         app_dirs.user_config_dir()?
         where loc = loc)
     end
-    match expected.site_config_dirs
+    match \exhaustive\ expected.site_config_dirs
     | _ExpectError =>
       h.assert_error(
         {()? => app_dirs.site_config_dirs()? }
@@ -111,7 +111,7 @@ primitive \nodoc\ _AppDirsTestUtil
         where loc = loc)
     end
 
-    match expected.user_cache_dir
+    match \exhaustive\ expected.user_cache_dir
     | _ExpectError =>
       h.assert_error(
         {()? => app_dirs.user_cache_dir()? }
@@ -123,7 +123,7 @@ primitive \nodoc\ _AppDirsTestUtil
         where loc = loc)
     end
 
-    match expected.user_state_dir
+    match \exhaustive\ expected.user_state_dir
     | _ExpectError =>
       h.assert_error(
         {()? => app_dirs.user_state_dir()? }
@@ -135,7 +135,7 @@ primitive \nodoc\ _AppDirsTestUtil
         where loc = loc)
     end
 
-    match expected.user_log_dir
+    match \exhaustive\ expected.user_log_dir
     | _ExpectError =>
       h.assert_error(
         {()? => app_dirs.user_log_dir()? }

appdirs/appdirs.pony

@@ -37,7 +37,7 @@ type _Maybe[T] is (T | None)
 
 primitive _Opt
   fun get[T](s: _Maybe[T], or_else: T): T =>
-    match consume s
+    match \exhaustive\ consume s
     | let value: T => consume value
     | None => consume or_else
     end
@@ -126,7 +126,7 @@ class AppDirs
     dir for this application.
     """
     let os_specific_dir =
-      match _platform
+      match \exhaustive\ _platform
       | _Osx =>
         Paths.join(
           [ _home as String
@@ -153,7 +153,7 @@ class AppDirs
           ""
         end
       end
-    match _app_version
+    match \exhaustive\ _app_version
     | None => os_specific_dir
     | let v: String => Path.join(os_specific_dir, v)
     end
@@ -175,7 +175,7 @@ class AppDirs
     data dirs for this application.
     """
     let os_specific_dirs: Array[String] iso =
-      match _platform
+      match \exhaustive\ _platform
       | _Osx =>
         recover
           [ Path.join(
@@ -239,7 +239,7 @@ class AppDirs
     for this application.
     """
     let os_specific_dir =
-      match _platform
+      match \exhaustive\ _platform
       | _Osx =>
         Paths.join(
           [ _home as String
@@ -254,7 +254,7 @@ class AppDirs
       end
 
     // apply version
-    match _app_version
+    match \exhaustive\ _app_version
     | None => os_specific_dir
     | let v: String =>
       Path.join(os_specific_dir, v)
@@ -275,7 +275,7 @@ class AppDirs
     for this application.
     """
     let os_specific_dirs: Array[String] iso =
-      match _platform
+      match \exhaustive\ _platform
       | _Osx =>
         recover
           [ Path.join(
@@ -326,7 +326,7 @@ class AppDirs
     for this application.
     """
     let os_specific_dir =
-      match _platform
+      match \exhaustive\ _platform
       | _Osx =>
         Paths.join(
           [ _home as String
@@ -351,7 +351,7 @@ class AppDirs
           ""
         end
       end
-    match _app_version
+    match \exhaustive\ _app_version
     | None => os_specific_dir
     | let v: String => Path.join(os_specific_dir, v)
     end
@@ -371,7 +371,7 @@ class AppDirs
     See:
     https://wiki.debian.org/XDGBaseDirectorySpecification#state
     """
-    match _platform
+    match \exhaustive\ _platform
     | _Osx | _Windows =>
       user_data_dir()?
     | _OsxAsUnix | _Unix =>
@@ -393,7 +393,7 @@ class AppDirs
     Return full path to the user-specific log dir
     for this application.
     """
-    match _platform
+    match \exhaustive\ _platform
     | _Osx =>
       Paths.join(
         [ _home as String