chore(flow): more comments&lint

[discord9]

I hereby agree to the terms of the GreptimeDB CLA.

Refer to a related PR or issue link (optional)

What's changed and what's your intention?

Just add bunch of comments and lints, also make function resolution for binary function carry correct signature now.

• lint for functions with too many lines
• comments for pub item
• make function resolution for binary function carry correct signature(i.e. Eq with i32 now have signature with input type i32)

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR does not require documentation updates.

[codecov]

Codecov Report

Attention: Patch coverage is 41.13924% with 93 lines in your changes are missing coverage. Please review.

Project coverage is 84.80%. Comparing base (883b7fc) to head (9e0fe1b).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3680      +/-   ##
==========================================
- Coverage   85.16%   84.80%   -0.36%     
==========================================
  Files         951      955       +4     
  Lines      158903   159099     +196     
==========================================
- Hits       135328   134931     -397     
- Misses      23575    24168     +593