prohibit reserved names

[yegor256]

Currently, I can do this:

[] > foo
  42 > true  

Here, I define a new attribute with the name true, which is the name of the object in the org.eolang package. I believe, it will be reasonable to throw warnings in this case. Let's prohibit all names that exist in the org.eolang package. Would be great to collect them during the build of the project, not define somewhere statically.