* Add optional second table argument to vim.json.decode which takes
  a table 'luanil' which can include the 'object' and/or 'array' keys. These
  options use luanil when converting NULL in json objects and arrays
  respectively. The default behavior matches the original lua-cjson.
* Remove recursive_convert_NIL function from rpc.lua, use
  vim.json.decode with luanil = { object = true } instead. This removes a hotpath
  in the json deserialization pipeline by dropping keys with json NULL
  values throughout the deserialized table.

* add vim.json.encode and vim.json.decode
* use vim.NIL instead of cjson.null
* resolve strict-prototypes warnings

* The following benchmark shows an approximately 2.5x (750 ms vs 300 ms) improvement in deserialization performance over
  vim.fn.json_decode on a medium package.json

  ```lua
  local uv = vim.loop
  local function readfile(path)
    return
  end

  local json_url = "https://raw.githubusercontent.com/rust-analyzer/rust-analyzer/b24c8d5c89ee93d1172b4127564f5da3b0c88dad/editors/code/package.json"
  io.popen(string.format('curl -v -f -L -O %q &> /dev/null', json_url))

  local json_string = io.open('package.json'):read '*a'

  uv.update_time()
  local start = uv.hrtime()

  for i = 1,1000 do
    vim.fn.json_decode(json_string)
  end

  uv.update_time()
  print(string.format("Deserialization time vim.fn.json_decode: %s ms", (uv.hrtime() - start) * (1e-6)))

  uv.update_time()
  local start = uv.hrtime()

  for i = 1,1000 do
    vim.json.decode(json_string)
  end
  uv.update_time()
  print(string.format("Deserialization time vim.json.decode: %s ms", (uv.hrtime() - start) * (1e-6)))
  ```

Co-Authored-By: Björn Linse <bjorn.linse@gmail.com>

Derived from the openresty lua-cjson fork at commit https://github.com/openresty/lua-cjson/commit/3d93d297092172eac3d52a1b3b6c1d479da5341a