* src/subst.ml: Rewrite completely, mostly following the λσ approach

* .gitignore: Add missing final newline.  Fix #*# pattern.

* src/lexp.ml (lexp): Swap Susp's arguments, to match the order usually
used in papers.  Adjust all users accordingly.

* src/typecheck.ml (mkSusp, scompose, sapply, ssink): New helpers to
use the new substitutions.
(conv_p'): Adjust to new substitutions's type.
(check): Check that Let's types are indeed types.  Fix compilation.