Refactor async mode to no longer use zpty

See technique used in `fast-syntax-highlighting`:
- ca2e18bbc9
- http://www.zsh.org/mla/users/2018/msg00424.html

Also see http://www.zsh.org/mla/users/2018/msg00432.html

In async response handler:
- We only want to read data in case of POLLIN or POLLHUP. Not POLLNVAL
  or select error.
- We always want to remove the handler, so it doesn't get called in an
  infinite loop when error is nval or err.

There is an upstream bug that prevents ctrl-c from resetting the prompt
immediately after a suggestion has been fetched asynchronously. A patch
has been submitted, but a workaround for now is to add `command true`
after the exec.

See https://github.com/zsh-users/zsh-autosuggestions/issues/364

spec/async_spec.rb

@@ -1,8 +1,4 @@
 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
@@ -31,52 +27,19 @@ context 'with asynchronous suggestions enabled' do
     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')
+  describe 'pressing ^C after fetching a suggestion' do
+    before do
+      skip 'Workaround does not work below v5.0.8' if session.zsh_version < Gem::Version.new('5.0.8')
+    end
 
-    session.clear_screen
+    it 'terminates the prompt and begins a new one' do
+      session.send_keys('e')
+      sleep 0.1
+      session.send_keys('C-c')
+      sleep 0.1
+      session.send_keys('echo')
 
-    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)')
+      wait_for { session.content }.to eq("e\necho")
     end
   end
 end