Illegal use of reserved keywords should be caught during contract analysis

[obycode]

There is a lot of discussion on this issue on the blockchain repo: stacks-network/stacks-core#3176

I will not duplicate all of it here, but the question is about whether the builtin functions and keywords should be reserved or if they can be freely used as variable and function names. Regardless of the answer to that question, we should ensure that the analysis-time behavior is the same as the run-time behavior.