Replace the buildTree() method by injected expr tracing approach

[h6ah4i]

Hi! I hope it will be a big step forward for runn and I'm so happy to introduce this improvement :tada:

What's new?

• Offers better pretty printing output in tree format for expr evaluation results.
• The new implementation is based on tracing approach. All evaluation results including intermediate states of the expr programs are preserved in memory, so each expr program is not be evaluated again during the pretty printing phase.
• The tracing feature is tested by the official expr testdata

Drawbacks

• Some performance overhead added due to tracing
• (breaking change) Removed YAML style # line comment handling. The behavior is changed in order to accept every valid syntax expr codes (the # symbol is a pointer/special variable marker in expr lang)

Behind the scenes

At first, I was trying to implement my proposal of the issue #803. However, soon I realized the current buildTree() method is not robust and there is no enough room for future development. That is why I decided to re-implement the pretty printing feature for expr evaluation results in completely new way.

[k1LoW]

@h6ah4i GREAT WORK!!!!!!!!! Thank you!!

First of all, thank you. I will now check the code.

However, soon I realized the current buildTree() method is not robust and there is no enough room for future development.

I agree with you.

(breaking change) Removed YAML style # line comment handling.

This was granted at a time when expr-lang did not have the ability to comment, and should go away. However, we cannot suddenly lose it, so I will try to be able to give a message that it is deprecated ( and replace ^# to // simply).