Use lexical-binding in all source files

[kazu-yamamoto]

Lexical binding is available since Emacs 24 but can be used in way that is compatible with Emacs 23. Tasks to make this happen:

1. Add ;;; -*- lexical-binding: t -*- to every *.el file.
2. Fix all warnings about unused lexical variables (be removing a variable or prepending underscore to its name).
3. Fix all errors about undefined lexical variables (if any).

Example of current output:

In toplevel form:
haskell-modules.el:51:1:Warning: Unused lexical argument `dontcreate'
haskell-modules.el:51:1:Warning: Unused lexical argument `session'
Wrote /Users/kazu/tmp/haskell-mode/haskell-modules.elc

[deepfire]

What might look like unused lexical binding can actually be a used dynamic binding..

[gracjan]

If this is the case then unused lexical binding should be converted into a declared and used dynamic binding.