compiler: frame-eliminating optimization.

This optimization identifies let blocks whose variables are
not captured by closures. The variables are relocated to
registers and the frame M N ... end reg wrapping is removed.

* parser.c (read_file_common): Load version 6 files.
We remain backwards-compatible.

* share/txr/stdlib/compiler.tl (var-spy, capture-var-spy): New
structure types.
(struct compiler): New slot, var-spies.
(with-var-spy): New macro.
(compiler (alloc-new-treg, unalloc-reg-count, push-var-spy,
pop-var-spy)): New methods.
(compiler (comp-atom, compt-setq, comp-list-setq,
comp-lisp1-value)): Inform the spies in the spy notification
stack about assignments and accesses.
(compiler eliminate-frame): New method.
(compiler comp-let): Use spies to determine which variables
from this frame are captured, and if none are, then use
eliminate-frame to rename all the variables to t-registers and
drop the frame setup/teardown.
(compiler comp-lambda): Set up a capture-var-spy which
intercepts accesses and assignments within a lambda, and
informs other spies about the captures.
(%tlo-ver%): Bump compiled file version to to (6 0), because
of some behavioral changes necessary in the VM. We might
revert this if the issues are solved differently.

* vm.c (vm_getz): Do not null out T registers.
(vm_execute_toplevel, vm_execute_closure): Use zalloca to
allocate the register part of the frame, so T registers are
initialized to nil.

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