Introduce mapped/Mapped

[mtfishman]

Introduce mapped/Mapped, which are analogous to Base.Broadcast.broadcasted/Base.Broadcast.Broadcasted for representing lazy maps.

The new preferred API for the package for converting from a broadcast expression to a map expression is:

using Base.Broadcast: broadcasted
using BroadcastMapConversion: Mapped

bc = broadcasted(*, 2, randn(2, 2))
m = Mapped(bc)
# Evaluate the map expression in multiple ways:
map(m.f, m.args...) == copy(bc)
copy(mapped(m.f, m.args...)) == copy(bc)
copy(m) == copy(bc)

Mapped is meant to support that same functionality as Broadcasted. I plan to deprecate the previous interface map_function(bc::Broadcasted) and map_args(bc::Broadcasted) in favor of the API above.

We might consider changing the package name to something like MapExpressions.jl, since converting to and from broadcast expressions can now be seen more as just one feature of the package and not necessarily the primary purpose. Also note the similarity to MappedArrays.jl, which provides a similar lazy map object, but that object is an AbstractArray while this one is a Base.AbstractBroadcasted so it serves a slightly different purpose and has a different interface.

[codecov]

Codecov Report

Attention: Patch coverage is 76.47059% with 4 lines in your changes missing coverage. Please review.

Project coverage is 88.37%. Comparing base (33096dc) to head (33f0363).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/BroadcastMapConversion.jl	76.47%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            main       #4       +/-   ##
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+ Coverage   0.00%   88.37%   +88.37%     
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  Files          1        1               
  Lines         24       43       +19     
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+ Hits           0       38       +38     
+ Misses        24        5       -19     

Flag	Coverage Δ
docs	88.37% <76.47%> (+88.37%)	⬆️

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