Add support for expanding singleton dimensions

[mtfishman]

Broadcasting in Julia supports expanding singleton dimensions to match a common size. Before this PR, when converting broadcasting expressions like:

julia> fill(1, 2, 2) .+ [1, 2]
2×2 Matrix{Int64}:
 2  2
 3  3

to map expressions, we didn't handle repeating over the singleton dimensions properly. This PR fixes that by creating map expressions that lazily repeat the smaller arrays using a combination of FillArrays.Fill and BlockArrays.BlockArray.

This PR also refactors some of the internals to hopefully make the logic a little clearer.

[codecov]

Codecov Report

Attention: Patch coverage is 96.34146% with 3 lines in your changes missing coverage. Please review.

Project coverage is 95.83%. Comparing base (3e63462) to head (c115549).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/BroadcastMapConversion.jl	96.34%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #6      +/-   ##
==========================================
+ Coverage   88.37%   95.83%   +7.46%     
==========================================
  Files           1        1              
  Lines          43       96      +53     
==========================================
+ Hits           38       92      +54     
+ Misses          5        4       -1     

Flag	Coverage Δ
docs	75.00% <71.95%> (-13.38%)	⬇️

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