remove Nim 1.2 workarounds and TODOs

[tersec]

All of these have been fixed:

• Poor performance when using return-value optimization due to double reset, nil check, genericReset nim-lang/Nim#13879
• Cannot borrow var float64 in infix assignment nim-lang/Nim#14440
• In for loop _ is accessible nim-lang/Nim#15972
• Underscore can not be used more than once as for loop variable nim-lang/Nim#16217

[github-actions]

Unit Test Results

         9 files  ±0    1 062 suites  ±0   30m 30s ⏱️ + 1m 8s
  3 514 tests ±0    3 277 ✔️ ±0  237 💤 ±0  0 ❌ ±0 
15 323 runs  ±0  15 058 ✔️ ±0  265 💤 ±0  0 ❌ ±0 

Results for commit 949a4d2. ± Comparison against base commit aacc8d7.