Abstract l/rmul and l/rdiv types

[jishnub]

This defines abstract types for LMul/RMul/LDiv/RDiv, and defines some of the methods for the supertypes rather than the specific types.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01% 🎉

Comparison is base (14721f1) 87.08% compared to head (9f96e09) 87.09%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #137      +/-   ##
==========================================
+ Coverage   87.08%   87.09%   +0.01%     
==========================================
  Files          10       10              
  Lines        1773     1775       +2     
==========================================
+ Hits         1544     1546       +2     
  Misses        229      229              

Files Changed	Coverage Δ
src/ldiv.jl	90.40% <100.00%> (+0.07%)	⬆️
src/lmul.jl	90.90% <100.00%> (+0.13%)	⬆️

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[dlfivefifty]

What's the motivation for this?

[jishnub]

To reduce the number of methods that need to be defined, which may improve the package-load and compile times slightly (although I haven't benchmarked these). From a design perspective, it seems redundant to define identical methods for every subtype.