* emacs/typer-mode.el: Render comments as markdown

(typer--markdown-verbatim-block-re, typer--markdown-verbatim-face):
New consts.
(typer--markdown-p): New function.
(typer-markdown-blockquote): New face.
(typer-markdown-keywords): New var.
(typer-font-lock-keywords): Use it.
(typer-mode): Setup use of `invisible' property.

* btl/pervasive.typer: Make use of new comment style.

* src/subst.ml: Rework top-level comment.