Implement grouped aggregations.

[xcharleslin]

• Fills out implementation of agg_groupby for Table in Rust. (The glue layers to the Python API are already done.)
• Adds a sort_grouper function for Table, which produces an argsort of the table, with identical values grouped together.
• Uses this to implement a grouped aggregation, which will go through each group, take on the indices, and then eval the aggregation expressions.
• Tests included. Tested multicolumn, multitype, empty grouped aggregations.

[codecov]

Codecov Report

Merging #704 (e35d185) into main (a31099d) will increase coverage by 0.15%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #704      +/-   ##
==========================================
+ Coverage   86.02%   86.18%   +0.15%     
==========================================
  Files         154      154              
  Lines       12589    12668      +79     
==========================================
+ Hits        10830    10918      +88     
+ Misses       1759     1750       -9     

Impacted Files	Coverage Δ
src/array/ops/sort.rs	99.30% <100.00%> (+<0.01%)	⬆️
src/table/ops/agg.rs	100.00% <100.00%> (+21.05%)	⬆️

... and 5 files with indirect coverage changes