Refactor checks and messages to be centered around code

[Fuco1]

All error messages should have a code. This way we can selectively ignore them with annotations or disable checks alltogether.

In general, there is a bigger refactoring necessary:

1. Each check should be its own class, including the "internal"
   checks, not only the rules from "rulesets". They do not have to be
   registered as-such, but should follow the same framework.
2. Each check should have richer metadata structure, similar to eslint:

• code => short descriptive code in lisp-naming-style-with-dashes
• level => notice, warning, error
• description => Human readable description, i.e. docstring
• messages => all the messages this check can produce. It's an
  alist from "code" to a format string. Format string should look
  like "The form ${form} can not accept type ${type}" (like
  s-lex-format). The placeholders will be extracted and supplied
  as plist values: pseudo code (make-message this form :id bla :form ... :name ...), where this is the check instance (maybe
  could be omited).
• schema: configuration schema. We should provide way to configure
  rules similar to eslint... but this is next step.