WIP: implement varmult_finite

[johnnychen94]

The implementation is more delicate than I thought it is; it is not very clean and there might be a lot of type-related issues in the current implementation.

I choose to keep the varfinite symbol as it's more convenient to use, if you think we should deprecate it then I can remove it.

Todo:

• check the type stability
• tests are needed

[codecov]

Codecov Report

Merging #26 (c59bc9b) into master (5687005) will decrease coverage by 0.47%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master      #26      +/-   ##
==========================================
- Coverage   92.07%   91.60%   -0.48%     
==========================================
  Files           5        5              
  Lines         227      250      +23     
==========================================
+ Hits          209      229      +20     
- Misses         18       21       +3     

Impacted Files	Coverage Δ
src/utils.jl	90.90% <66.66%> (-9.10%)	⬇️
src/statistics.jl	85.41% <93.33%> (+3.27%)	⬆️

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