Hello,

The attached patch is an attempt to add unused variable reporting to the
compiler, at the GLIL->assembly step.  It was quite simple to implement
here, and it should work with all front-ends (Scheme, ECMAScript, etc.),
which is nice.

Example:

--8<---------------cut here---------------start------------->8---
scheme@(guile-user)> (lambda () (let ((x 2)) x))
$1 = #<program 7ffae3f42b40 at <unknown port>:1:0 ()>
scheme@(guile-user)> (lambda () (let ((x 2)) 'something-else))
<stdin>:3:11: variable `x' never referenced
$2 = #<program 7ffae3f36560 at <unknown port>:2:0 ()>
--8<---------------cut here---------------end--------------->8---

It works as well on actual programs such as the compiler itself, but
leads to a stack overflow on files with lots of unused variables, such
as `language/tree-il/compile-glil.scm', although `analyze-program' is
tail-recursive AFAICS (ideas welcome).

Also, it's annoying because it reports unused bindings introduced by
`record-case', and there are lots of them in the compiler.

Is this the right place and the right way to do such things?  Comments?

Thanks,
Ludo'.