Remove MEM_PROFILE related code

Code around `#ifdef MEM_PROFILE` was used to profile vim's memory
comsumption. It's very likely broken as new code is using malloc() and free()
directly.

In this day and age, valgrind can solve in a much reliable way what
this code was trying to do.
HAVE_AVAIL_MEM is always undefined. Remove tests and dead code

Defining HAVE_AVAIL_MEM doesn't even build. The code tries to call
`mch_avail_mem` which is not defined.
Use realloc instead of vim_realloc
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