Replace `lexical-let' with lexical-binding `let'

* sly.el (sly-rex, sly-minibuffer-setup-hook, sly-edit-value)
(sly-edit-value-commit, sly-eval-macroexpand-inplace)
(sldb-recompile-frame-source, sly-inspector-reinspect)
(sly-eval-feature-expression): Replace `lexical-let' with let.
Some preparatory work for multiple inspectors

`sly-inspector-mark-stack' and its functionality is gone. `sly-mrepl'
and other asynchronous inspection requests from Emacs have to be
analysed still.

* sly.el (sldb-local-variable, sldb-inspect-in-frame)
(sldb-inspect-condition, sly-inspect, sly-inspector-part)
(sly-inspector-insert-ispec, sly-inspector-pop)
(sly-inspector-reinspect, sly-inspector-toggle-verbose): Use
`sly-eval-for-inspector'
(sly-eval-for-inspector): New function.
(sly-inspector-mark-stack): Remove.
(sly-inspector-buffer): Don't set `sly-inspector-mark-stack'

* contrib/sly-trace-dialog.el (sly-trace-dialog-part)
(sly-trace-dialog-spec): Use `sly-eval-for-inspector'

* lib/lisp/swank.lisp (eval-for-inspector): New slyfun.