Proper support for RESP3 PUSH messages.

By default, PUSH messages are now intercepted and the reply memory freed.  
This means existing code should work unchanged when connecting to Redis
>= 6.0.0 even if `CLIENT TRACKING` were then enabled.

Additionally, we define two callbacks users can configure if they wish to handle
these messages in a custom way:

void redisPushFn(void *privdata, void *reply);
void redisAsyncPushFn(redisAsyncContext *ac, void *reply);

See #825

* Adds an indirection to every allocation/deallocation to allow users to 
  plug in ones of their choosing (use custom functions, jemalloc, etc).

* Gracefully handle OOM everywhere in hiredis.  This should make it possible
  for users of the library to have more flexibility in how they handle such situations.

* Changes `redisReaderTask->elements` from an `int` to a `long long` to prevent
  a possible overflow when transferring the task elements into a `redisReply`.

* Adds a configurable `max elements` member to `redisReader` that defaults to
  2^32 - 1.  This can be set to "unlimited" by setting the value to zero.

Addresses #506

Housekeeping

* Check for C++ (#758, #750) 
* Include `alloc.h` in `make install` and `cmake`
* Add a `.def` file for Windows (#760)
* Include allocation wrappers referenced in adapter headers
* Fix minor syntax errors and typos in README
* Fix CI in Windows by properly escaping arguments (#761)

 - Removed whitespace before newline
 - Removed win style newline

Add note about using context->data to pass user data to connect and disconnect callbacks

[ci-skip]

Closes #361

Closes #267

Closes #285

Closes #293

Closes #121.

Updated documentation and examples accordingly.

Hiredis used to free unused redisReader buffers bigger than 16k. Now
this limit is configurable (see the documentation updated by this commit)
in order to allow working with big payloads without incurring to speed
penalty.

See issue #79, thanks to @ewencp.