Reimplement input functions on top of libuv

The functions `mch_inchar`, `mch_breakcheck`, `mch_char_avail` were
reimplemented on top of libuv. Here's how it works:

- When Neovim needs to wait for characters, it will transfer control to libuv
  event loop.
- When the libuv event loop gets user input, it will transfer control back to
  Neovim
- Neovim uses the `input_read` function to get the actual data read by libuv.

With this scheme its possible to keep Neovim single-threaded while enjoying the
benefits provided by libuv.

This commit leaves SIGWINCH broken for now

src/os/event.c

@@ -0,0 +1,74 @@
+#include <stdint.h>
+#include <stdbool.h>
+
+#include <uv.h>
+
+#include "os/event.h"
+#include "os/input.h"
+
+static uv_timer_t timer_req;
+static void timer_cb(uv_timer_t *handle, int);
+
+void event_init()
+{
+  /* Initialize input events */
+  input_init();
+  /* Timer to wake the event loop if a timeout argument is passed to
+   * `event_poll` */
+  uv_timer_init(uv_default_loop(), &timer_req);
+}
+
+/* Wait for some event */
+EventType event_poll(int32_t ms)
+{
+  bool timed_out;
+  EventType event;
+  uv_run_mode run_mode = UV_RUN_ONCE;
+
+  if ((event = input_check()) != kEventNone) {
+    /* If there's a pending input event to be consumed, do it now */
+    return event;
+  }
+
+  input_start();
+  timed_out = false;
+
+  if (ms > 0) {
+    /* Timeout passed as argument, start the libuv timer to wake us up and 
+     * set our local flag */
+    timer_req.data = &timed_out;
+    uv_timer_start(&timer_req, timer_cb, ms, 0);
+  } else if (ms == 0) {
+    /* 
+     * For ms == 0, we need to do a non-blocking event poll by
+     * setting the run mode to UV_RUN_NOWAIT.
+     */
+    run_mode = UV_RUN_NOWAIT;
+  }
+
+  do {
+    /* Wait for some event */
+    uv_run(uv_default_loop(), run_mode);
+  } while (
+      /* Continue running if ... */
+      (event = input_check()) == kEventNone && /* ... we have no input */
+      run_mode != UV_RUN_NOWAIT && /* ... ms != 0 */
+      !timed_out  /* ... we didn't get a timeout */
+      );
+
+  input_stop();
+
+  if (!timed_out && ms > 0) {
+    /* Timer event did not trigger, stop the watcher since we no longer
+     * care about it */
+    uv_timer_stop(&timer_req);
+  }
+
+  return event;
+}
+
+/* Set a flag in the `event_poll` loop for signaling of a timeout */
+static void timer_cb(uv_timer_t *handle, int status)
+{
+  *((bool *)handle->data) = true;
+}