Changes to be committed:
* README.md: minor updates
* src/lparse.ml:
	- add an error message when lambda does not habe type info
	- fix lexp.Arrow, its variable was not added to the environement
* src/builtin.ml
* btl/macro.typer:
	- added some ideas on how to define type built-in
* src/lexp.ml:
	- fix lexp_print Inductive's formal args