compiler: treat nested load-time forms.

load-time forms nested in load-time forms have no special
semantics; it's wasteful to separately hoist them into load
time and store their value in their own D register.
We must be careful: this is not true if a nested form occurs
in a lambda.

* share/txr/stdlib/compiler.tl (*load-time*): New special
variable.
(compiler comp-lambda): Bind *load-time* to nil around
the compilation of the lambda, so load-time forms in the
lambda are hoisted to load time, even if the lambda itself
is wrapped in a load-time form.
(compiler comp-load-time-lit): Bind *load-time* true around
the compilation of the form. If *load-time* is already true,
then skip the special load-time logic and just compile the
enclosed form; the surrounding load-time compilation is taking
care of the load-time hoisting.

* txr.1: Document that load-time forms nested in load-time
forms don't do anything, except in the lambda case.

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