Optimize rpc msg parse loop

[mfussenegger]

With a script like:

local num_bodies = 0
local function handle_body(_)
  num_bodies = num_bodies + 1
end
local function on_eof()
end
local parse_loop = require("dap.rpc").create_read_loop(handle_body, on_eof)

local payload = {
  seq = 1,
  type = "event",
  body = {
    foo = "bar"
  }
}
local msg = require("dap.rpc").msg_with_content_length(vim.json.encode(payload))

local pipes = vim.uv.pipe()

local stdin = assert(vim.uv.new_pipe())
local stdout = assert(vim.uv.new_pipe())
stdin:open(pipes.write)
stdout:open(pipes.read)
local iterations = 50000
local start = vim.uv.hrtime()
for i = 1, iterations do
  stdin:write(msg)
end
stdin:write("", function()
  stdin:close()
end)
stdout:read_start(parse_loop)

vim.wait(5000, function() return num_bodies == iterations end)
local duration = vim.uv.hrtime() - start
vim.print({ duration = duration * 0.000001, iterations = num_bodies })

Before: 130.322467
After: 74.878325