N2O for Lean 4

Build

UNIX-like OS:

1. First, build Lean 4:

   $ git clone https://github.com/leanprover/lean4
   $ mkdir lean4/build
   $ cd lean4/build
   $ cmake ../src
   $ make

2. Then build libwebsockets:

   $ git clone https://github.com/warmcat/libwebsockets
   $ mkdir libwebsockets/build
   $ cd libwebsockets/build
   $ cmake ..
   $ make
   $ sudo make install

3. Install BUM.

4. And:

   $ git clone https://github.com/o89/n2o/
   $ cd n2o
   $ bum compile