n-api: add API for asynchronous functions

Bundle a `uv_async_t`, a `uv_idle_t`, a `uv_mutex_t`, a `uv_cond_t`,
and a `v8::Persistent<v8::Function>` to make it possible to call into JS
from another thread. The API accepts a void data pointer and a callback
which will be invoked on the loop thread and which will receive the
`napi_value` representing the JavaScript function to call so as to
perform the call into JS. The callback is run inside a
`node::CallbackScope`.

A `std::queue<void*>` is used to store calls from the secondary
threads, and an idle loop is started by the `uv_async_t` callback on the
loop thread to drain the queue, calling into JS with each item.

Items can be added to the queue blockingly or non-blockingly.

The thread-safe function can be referenced or unreferenced, with the
same semantics as libuv handles.

Re: nodejs/help#1035
Re: #20964
Fixes: #13512
PR-URL: #17887
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

Author: Gabriel Schulhof

doc/api/errors.md

@@ -1376,6 +1376,31 @@ multiple of the element size.
 While calling `napi_create_typedarray()`, `(length * size_of_element) +
 byte_offset` was larger than the length of given `buffer`.
 
+<a id="ERR_NAPI_TSFN_CALL_JS"></a>
+### ERR_NAPI_TSFN_CALL_JS
+
+An error occurred while invoking the JavaScript portion of the thread-safe
+function.
+
+<a id="ERR_NAPI_TSFN_GET_UNDEFINED"></a>
+### ERR_NAPI_TSFN_GET_UNDEFINED
+
+An error occurred while attempting to retrieve the JavaScript `undefined`
+value.
+
+<a id="ERR_NAPI_TSFN_START_IDLE_LOOP"></a>
+### ERR_NAPI_TSFN_START_IDLE_LOOP
+
+On the main thread, values are removed from the queue associated with the
+thread-safe function in an idle loop. This error indicates that an error
+has occurred when attemping to start the loop.
+
+<a id="ERR_NAPI_TSFN_STOP_IDLE_LOOP"></a>
+### ERR_NAPI_TSFN_STOP_IDLE_LOOP
+
+Once no more items are left in the queue, the idle loop must be suspended. This
+error indicates that the idle loop has failed to stop.
+
 <a id="ERR_NO_CRYPTO"></a>
 ### ERR_NO_CRYPTO