Improvements to stdlib documentation

While working on #294, I thought of a few potential improvements to the automatic documentation generation we have for stdlib. To list a few of them:

* We should probably introduce a new type of comment for documentation comments to make parsing simpler. Maybe something like `#!`.
* One thing that would be good to have is a way to specify some structured info about procedures. For example, whether a procedure is `unstable`, how many cycles a procedure takes to execute, if a procedure belongs to a specific category (e.g. "arithmetic operations" for u64 module). One way to do it is via some attributes within comments - e.g., something like `@cycles`. Another way could be general attributes for procedures (similar to how Rust does it).
* Support Markdown within comments (similar to Rust) so that we could have nice-looking procedure descriptions. This would also mean that we'd need to move away from outputting procedure docs as a single big table.