Notably faster fromhex()

[nnposter]

Function stdnse.fromhex() is used extensively throughout the NSE codebase, which makes it worth optimizing. The following drop-in replacement performs about 43% faster[1] and creates only a quarter of temporary Lua strings than the current implementation:

function fromhex (hex)
  local p = hex:find("[^%x%s]")
  if p then
    return nil, "Invalid hexadecimal digits at position " .. p
  end
  hex = hex:gsub("%s+", "")
  if #hex % 2 ~= 0 then
    return nil, "Odd number of hexadecimal digits"
  end
  return hex:gsub("..", function (h) return string.char(tonumber(h, 16)) end)
end

I will commit the change in a few weeks unless concerns are raised.

[1] https://gist.github.com/nnposter/b9e4e1a9b9d2637f8c8c6b2948866640

[dmiller-nmap]

I like it! Thanks for taking the time to work it out. I don't see any issues.

[nnposter]

Committed as r37938 (f5b4d98)