compiler/vm: more compact frame size for closures.

Closures do not share t-registers with surrounding code; they
do not store a value into such a register that code outside
the closure would read and vice versa.

When compiling closures, we can can temporarily reset the
compiler's t-register allocator machinery to get low
t-register values.  Then, when executing the closure, we
reserve space just for the registers it needs, not based off
the containing vm description.

Here we make a backwards-incompatible change. The VM close
instruction needs an extra parameter indicating the number of
t-regisers. This is stored into the closure and used for
allocating the frame when it is dispatched.

* parser.c (read_file_common): We read nothing but version 6
tlo files now.

* share/txr/stdlib/asm.tl (op-close asm): Parse new ntreg
argument from close syntax, and put it out as an extra word.
Here is where we pay for this improvement in extra code size.
(op-close dis): Extract the new argument from the machine code
and add it to the disassembled format.

* share/txr/stdlib/compiler.tl (compile-in-toplevel): Save and
restore the t-reg discards list also. Don't bother with a
gensym for the compiler; the argument is always a symbol,
which we can use unhygienically like in with-var-spy.
(compile-with-fresh-tregs): New macro based on
compile-in-toplevel: almost the same but doesn't reset the
level.
(comp-lambda-impl): Use compile-with-fresh-tregs to compile
the entire closure with a minimized register set.
Place the treg-cntr into the closure instruction to indicate
the number of registers the closure requires.

* vm.c (struct vm): New member, nreg.
(vm_make_closure): New parameter, nreg, stored into the
closure.
(vm_close): Extract a third opcode word, and pull the nreg
value from the bottom half. Pass this to vm_make_closure.
(vm_execute_closure, vm_funcall_common): Calculate frame size
based on the closur's nreg rather than the VM description's.

* txr.1: Document that the upcoming version 252 produces
version 6.0 object files and only loads version 6.

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