feat: add realm /r/demo/math_eval

[schollz]

This eval realm is capable of evaluating 32-bit integer expressions as they would appear in Go. For example, you can use this package (and realm) to evaluate an expression like "(32*32+4)-1" when presented as a string.

This could have several applications. This current "eval" realm is simply demonstrating it as a 32-bit integer calculator.

I would like to also add floats, but it seems strconv.ParseFloat is not available yet.

In the future I would like to use the int32 version of this package in the bytebeat realm (#840) which could then utilize this library to generate audio directly from string expressions, instead of programmed realms - which ultimately gets closer to an audio NFT platform where expressions are uploaded if they are parsed correctly.

To test, you can just add the eval package and realm:

> gnokey maketx addpkg --pkgpath "gno.land/p/demo/math_eval_int32" \
    --pkgdir "examples/gno.land/p/demo/math_eval/int32" \
    --deposit 100000000ugnot --gas-fee 1000000ugnot \
    --gas-wanted 2000000 --broadcast --chainid dev --remote localhost:26657 \
    YOURKEY
> gnokey maketx addpkg --pkgpath "gno.land/r/demo/math_eval" \
    --pkgdir "examples/gno.land/r/demo/math_eval" \
    --deposit 100000000ugnot --gas-fee 1000000ugnot \
    --gas-wanted 2000000 --broadcast --chainid dev --remote localhost:26657 \
    YOURKEY

Then open up to localhost:8888/r/demo/math_eval and try expressions in the URL query, like localhost:8888/r/demo/math_eval:1+1.

Checklists...

Contributors Checklist

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests
• Added new benchmarks to generated graphs, if any. More info here.

Maintainers Checklist

• Checked that the author followed the guidelines in CONTRIBUTING.md
• Checked the conventional-commit (especially PR title and verb, presence of BREAKING CHANGE: in the body)
• Ensured that this PR is not a significant change or confirmed that the review/consideration process was appropriate for the change

[moul]

Can you rename this realm to something else than « eval », maybe « bc » to match the Unix utility, or « math_eval ». Up to you.

[schollz]

@moul yes, no problem! math_eval is good - clear and concise.

[harry-hov]

I miss gno.mod files :)

[moul]

I miss gno.mod files :)

@harry-hov, we need an explicit CI check for this.

Edit: started working on it here: #981