Add warning when local macros used in attr expression.

The with-html-output ("WHO") macro defines some local macrolets.
Those are in scope of the entire thing, but they can only be
meaningfully used in the imperative Lisp forms that are placed
into the HTML tag body. If they are used in an attribute
expression, it's probably a programming error; an output
side effect doesn't belong there.  We implement a diagnostic
strategy which produces a warning in this situation.

* who.tl (attr-warning-macrolet): New function, which
wraps macrolet code around the expression passed in.
These macrolets intercept the local macros and generate
a warning. Then they decline to expand, deferring to the
real macros.
(convert-attributes): Wrap the warning macrolets around
the run-time evaluation of the val expression. We don't
do anything for the compile-time evaluation of a constant
attribute expression, because the macro calls we want
to intercept do not appear in a constant expression.

* test/simple.tl: New test 41 validating that warnings
are generated for all the local macros when they are called
from an attr expression.

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