cx playground: maximum time for execution of script before it fails

[SkycoinSynth]

If a person submits a CX program to compile

• we need to time how long execution takes
• if execution exceeds a threshold, then we need to abort execution (and close that thread/stop compilation)
• "timeout" should default to 5000 milliseconds
• timeout needs to be a command line parameter
• must return error "execution time out, program took more than %N milliseconds to execute"

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Timeout can be for compiling program

• report how long program took to compile/parse

Timeout can be execution of program

• report how long program took to execute

The same timeout can be used for total time (both compilation and execution).