Correct check for jump outside exception for break and continue.
Correct calculation of first varargs stack offset.
Add missing debug output for new math operations.
Add code to reenter vm from an exception in C code.
Zero stack memory if thrown exception leaves a vm frame.

  The change partially mimics the code in function epilogs, to zero
the stack memory, so that "garbage" would not be confused with a valid
pointer when the stack layout changed in another function call.
Add a new test file with initial math tests.

  This change also adds proper "+ num" implementation, that was
delayed on purpose, to debug the handling of exceptions being
generated in a frame other than the one handling it; in this case,
exceptions generated in the "check" function, but set and handled
in the "unary" function of the new check/math test.
Correct scan_chr to avoid problems when reading wordsize bytes.