`try`/`catch`'s behavior can be easily overridden

[thacuber2a03]

xpcall = (...) -> print 'get hacked idiot'
pcall = xpcall

this changes completely what try/catch does, at least until the end of the current scope, though if this is done at top-level it's impossible to change back
I'd propose accessing the global scope and those functions explicitly when compiling a try/catch:

try
  func 1, 2, 3
catch err
  print yue.traceback err

try
  print "trying"
  func 1, 2, 3

->

_G.xpcall(function() -- 1
  return func(1, 2, 3) -- 2
end, function(err) -- 2
  return print(yue.traceback(err)) -- 4
end) -- 1

_G.pcall(function() -- 17
  print("trying") -- 18
  return func(1, 2, 3) -- 19
end) -- 17

sure, it won't completely block this behavior; anyone who decides to write _G.xpcall = (...) -> is going to invoke this problem, but at least it should be actively harder to do on accident. you shouldn't really be overwriting the protected call functions unless you know precisely what you'll do with them

it's also not the worst thing ever, this is more of a small fix than anything

[thacuber2a03]

in retrospect, it'd be better to "freeze" pcall and xpcall somewhere and use those frozen versions; something like

-- at the top of the file:
_yue_xpcall = rawget(_G, 'xpcall')
_yue_pcall = rawget(_G, 'pcall')

-- later:
_yue_xpcall(function() -- 1
  return func(1, 2, 3) -- 2
end, function(err) -- 2
  return print(yue.traceback(err)) -- 4
end) -- 1

this can still be sidestepped as well, but, it's not really possible to completely lock this behavior, and it isn't a good idea to do so either