msgpack-rpc: Create subdirectory for msgpack-rpc modules

Create the msgpack_rpc subdirectory and move all modules that deal with
msgpack-rpc to it. Also merge msgpack_rpc.c into msgpack_rpc/helpers.c
channel: Simplify resource management

- Remove unused rpc_call_level field
- Add `returned` field to ChannelCallFrame. This is set when the call returns
  and is the only condition checked by `channel_send_call`.
- Add job_exit callback for properly closing channels created from job(the
  job_exit callback is only called after all read callbacks, so it's the only
  safe place to free the channel).
event: Remove EventType enum and pass a callback to `event_push`

This approach is more flexible because we don't need to support a fixed set of
"event types", any module can push events to be handled in main loop by simply
passing a callback to the Event structure.
event: Remove automatic event deferall

This is how asynchronous events are currently handled by Nvim:

- Libuv event loop is entered when Nvim blocks for user input(os_inchar is
  called)
- Any event delivered by libuv that is not user input is queued for processing
- The `K_EVENT` special key code is returned by os_inchar
- `K_EVENT` is returned to a loop that is reading keys for the current Nvim
  mode, which will be handled by calling event_process()

This approach has the advantage of integrating nicely with the current
codebase, eg: vimscript code can be executed asynchronously with little
surprises(Its the same as if the user typed a key).

The problem with using keys to represent any event is that it also interferes with
operators, and not every event needs or should do that. For example, consider
this scenario:

- A msgpack-rpc client calls vim_feedkeys("d")
- Nvim processes K_EVENT, pushing "d" to the input queue
- Nvim processes "d", entering operator-pending mode to wait for a motion
- The client calls vim_feedkeys("w"), expecting Nvim to delete a word
- Nvim processes K_EVENT, breaking out of operator-pending and pushing "w"
- Nvim processes "w", moving a word

This commit fixes the above problem by removing all automatic calls to
`event_push`(which is what generates K_EVENT input). Right now this also breaks
redrawing initiated by asynchronous events(and possibly other stuff too, Nvim is
a complex state machine and we can't simply run vimscript code anywhere).

In future commits the calls to `event_push` will be inserted only where it's
absolutely necessary to run code in "key reading loops", such as when executing
vimscript code or mutating editor data structures in ways that currently can
only be done by the user.
event: Extract event_poll loops to `event_poll_until` macro

A pattern that is becoming common across the project is to poll for events until
a certain condition is true, optionally passing a timeout. To address this
scenario, the event_poll_until macro was created and the job/channel/input
modules were refactored to use it on their blocking functions.
eval: Defer execution of JobActivity autocommands

JobActivity autocommands run vimscript and must be executed on Nvim main loop.
Since the previous commit removed automatic calls to `event_push` on RStream/Job
callbacks, this adds it back, but in eval.c where job control is implemented.
msgpack-rpc: Allow selective deferral API calls

Since all API functions now run immediately after a msgpack-rpc request is
parsed by libuv callbacks, a mechanism was added to override this behavior and
allow certain functions to run in Nvim main loop.

The mechanism is simple: Any API function tagged with the FUNC_ATTR_DEFERRED (a
"dummy" attribute only used by msgpack-gen.lua) will be called when Nvim main
loop receives a K_EVENT key.

To implement this mechanism it was necessary some restructuration on the
msgpack-rpc modules, especially in the msgpack-gen.lua script.
api: Add FUNC_ATTR_DEFERRED attribute to a number of functions

Any function that can directly mutate the screen or execute vimscript had the
attribute applied.
compilation: Add -Wconversion to more files and validate CONV_SOURCES

All files under the os, api and msgpack_rpc directories have -Wconversion
automatically applied. CONV_SOURCES is also checked for missing files(when
renaming, for example)
fix channel_set_error(squash to channel: Simplify..)