metaticket: remaining tasks from src/sage/symbolic/todo.txt

[gh-DaveWitteMorris]

The file src/sage/symbolic/todo.txt is removed by #30482. Here are the only tasks that remain to be completed:

• genuine coercions to real field, etc.
• optimize is_even in numeric.cpp
• Support pickle via the "archive" print mode.
• need to be able to do this (from ginsh):

  > collect_common_factors(x/(x^2 + x));
  (1+x)^(-1)
  

• Maybe change Sage's GiNaC to make a call to a Cython gcd function, then use Singular, since Singular's gcd over QQ is much better than GiNaC's, I think, and GiNaC *only* does GCD over QQ. Actually, just make everything in normal.cpp be implemented via Singular, probably...

Probably some of these can be deleted as invalid, but some may need tickets.

See also:

• ​https://trac.sagemath.org/wiki/symbolics