Adds to the existing debug module

_DEBUG is either any true value (equivalent to {level = 1}), or a
table with the following members:

level: debugging level
call: do call trace debugging
std: do standard library debugging (run examples & test code)

say: Print a debugging message
  [n]: debugging level [1]
  ...: objects to print (as for print)

Expose say as global debug

traceCall: Trace function calls
  event: event causing the call
Use: debug.sethook (traceCall, "cr"), as below
based on test/trace-calls.lua from the Lua 5.0 distribution

Set hooks according to _DEBUG