Revert async process substitution & completion strategy

They're not quite ready. Keep them on a feature branch for now.

spec/async_spec.rb

@@ -1,4 +1,8 @@
 context 'with asynchronous suggestions enabled' do
+  before do
+    skip 'Async mode not supported below v5.0.8' if session.zsh_version < Gem::Version.new('5.0.8')
+  end
+
   let(:options) { ["ZSH_AUTOSUGGEST_USE_ASYNC="] }
 
   describe '`up-line-or-beginning-search`' do
@@ -26,6 +30,55 @@ context 'with asynchronous suggestions enabled' do
       end
     end
   end
+
+  it 'should not add extra carriage returns before newlines' do
+    session.
+      send_string('echo "').
+      send_keys('escape').
+      send_keys('enter').
+      send_string('"').
+      send_keys('enter')
+
+    session.clear_screen
+
+    session.send_string('echo')
+    wait_for { session.content }.to eq("echo \"\n\"")
+  end
+
+  it 'should treat carriage returns and newlines as separate characters' do
+    session.
+      send_string('echo "').
+      send_keys('C-v').
+      send_keys('enter').
+      send_string('foo"').
+      send_keys('enter')
+
+    session.
+      send_string('echo "').
+      send_keys('control').
+      send_keys('enter').
+      send_string('bar"').
+      send_keys('enter')
+
+    session.clear_screen
+
+    session.
+      send_string('echo "').
+      send_keys('C-v').
+      send_keys('enter')
+
+    wait_for { session.content }.to eq('echo "^Mfoo"')
+  end
+
+  describe 'exiting a subshell' do
+    it 'should not cause error messages to be printed' do
+      session.run_command('$(exit)')
+
+      sleep 1
+
+      expect(session.content).to eq('$(exit)')
+    end
+  end
 end