unwrap j=list(var) for constants

[ben-schwen]

Closes #712
Should probably follow #7401

Timings from issue

DT <- data.table(x=rep(1:1e7, 2L), y=1L)
microbenchmark::microbenchmark(times = 50L, DT[, .N, by=x], DT[, list(N = .N), by=x])
# Unit: milliseconds
#                        expr      min       lq     mean   median       uq      max neval
#            DT[, .N, by = x] 251.0772 284.3042 297.7992 293.8167 306.1571 356.7400    50
#  DT[, list(N = .N), by = x] 245.8979 286.3050 299.6035 295.5314 304.7920 369.8054    50

dd <- data.table(x=1:1e6,key="x")
microbenchmark::microbenchmark(times = 50L, 
    dd[dd, 1, by = .EACHI],
    dd[dd, list(1), by = .EACHI]
)

# Unit: milliseconds
#                          expr      min       lq     mean   median       uq      max neval
#        dd[dd, 1, by = .EACHI] 203.6271 208.2239 212.7361 209.5664 217.1072 241.5754    50
#  dd[dd, list(1), by = .EACHI] 204.4147 206.8032 212.0772 210.9118 215.1693 241.4883    50

microbenchmark::microbenchmark(times = 50L, 
    dd[dd, x, by = .EACHI],
    dd[dd, list(x), by = .EACHI]
)

# Unit: milliseconds
#                          expr      min       lq     mean   median       uq      max neval
#        dd[dd, x, by = .EACHI] 259.9601 263.3664 271.2088 267.9141 278.3005 290.7164    50
#  dd[dd, list(x), by = .EACHI] 257.3253 264.2222 272.0825 270.7082 277.3748 307.8739    50

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.00%. Comparing base (0216983) to head (7f8dbdd).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7565   +/-   ##
=======================================
  Coverage   99.00%   99.00%           
=======================================
  Files          87       87           
  Lines       16893    16902    +9     
=======================================
+ Hits        16725    16734    +9     
  Misses        168      168           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[github-actions]

• HEAD=optimize_listVar slower P<0.001 for memrecycle regression fixed in #5463
  

Generated via commit 079f000

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	2 minutes and 48 seconds
Installing different package versions	43 seconds
Running and plotting the test cases	4 minutes and 4 seconds