JOB_STACKFRAME_NORMAL

  job_RunState.l   ->   size in bytes of stackframe
                        JOB_STACKFRAME_NORMAL   
                        local_variable_N
			...
			local_variable_1
  job_RunState.v   ->	local_variable_0
                        compiledFunction for this frame.
                        programCounter for this frame.
                        size in bytes of stackframe

where job_RunState.l is an internal interpreter variable pointing to the current top of loop stack, and job_RunState.v is an internal interpreter variable pointing to the currently active set of local variables.

Rationale:

• We need an explicit stackframe type so THROW, ERROR etc can walk down the loop stack looking for what they want;
• We need to save 'x_obj' and 'pc' so we can continue execution at the correct bytecode upon return to function;
• We need size of stack frame at both top and bottom so as to be able to traverse the stack either direction. The given size includes both top and bottom sizewords, and the size is stored as a straight C integer, not a Muq-encoded one. (This is slightly unclean, but is safe because the stackframe sizes will all be a multiple of sizeof(Vm_Obj) -- four bytes on 32-bit machines -- and hence will have the least significant bit zero and thus will look like a Muq integer to any Muq code inspecting the size slots. This slight uncleanliness buys us the ability to walk down the stack at maximum plausible speed.)

Note: The very bottom stackframe on a stack has a zero word below it. This stops any attempt to traverse down past the bottom of the stack:

  job_RunState.l   ->   size in bytes of below stackframe
                        JOB_STACKFRAME_NORMAL   
                        local_variable_N
			...
			local_variable_1
  job_RunState.v   ->	local_variable_0
                        compiledFunction for this frame.
                        programCounter for this frame.
                        size in bytes of above stackframe
                        ZERO WORD

A full getStackframe[ NORMAL frame result looks like:

:owner             object
:kind              :normal
:programCounter   fixnum
:compiledFunction object
:variables         count of local variables
0                  value of local_variable_0
1                  value of local_variable_1
...                ...

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