refactor(rust): Add var/std support to new streaming engine

[orlp]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 2.40964% with 243 lines in your changes missing coverage. Please review.

Project coverage is 79.96%. Comparing base (4cfe56f) to head (a6159ea).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-expr/src/reduce/var_std.rs	0.00%	102 Missing ⚠️
crates/polars-expr/src/reduce/mod.rs	0.00%	59 Missing ⚠️
crates/polars-expr/src/reduce/min_max.rs	0.00%	42 Missing ⚠️
crates/polars-expr/src/reduce/mean.rs	0.00%	26 Missing ⚠️
crates/polars-compute/src/var_cov.rs	37.50%	10 Missing ⚠️
crates/polars-expr/src/reduce/convert.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19406      +/-   ##
==========================================
- Coverage   80.04%   79.96%   -0.08%     
==========================================
  Files        1532     1533       +1     
  Lines      210713   210877     +164     
  Branches     2442     2442              
==========================================
- Hits       168658   168626      -32     
- Misses      41500    41696     +196     
  Partials      555      555              

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