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

Makefile

@@ -4,7 +4,6 @@ SRC_FILES := \
 	$(SRC_DIR)/setup.zsh \
 	$(SRC_DIR)/config.zsh \
 	$(SRC_DIR)/util.zsh \
-	$(SRC_DIR)/features.zsh \
 	$(SRC_DIR)/bind.zsh \
 	$(SRC_DIR)/highlight.zsh \
 	$(SRC_DIR)/widgets.zsh \