Remove `eval_str` and use of `eval`

[mattsignorelli]

We should avoid eval and calling functions for code generation, as it is not done statically. See the discussion here

[DavidSagan]

Yes, in general meta-programming should be avoided. However there are some limited situation where its use is warranted. This needs to be discussed on a case-by-case basis. The only place eval_str is used currently is for enums and I don't see that as a problem.

[mattsignorelli]

I am not sure yet about the necessity of metaprogramming, but this issue is solely about using the function enum + eval to do metaprogramming, instead of defining a macro (e.g. macro make_enum) and using that to do the metaprogramming. If you use a macro it is done statically which is better