add conversion for map to object

[jbardin]

Allow conversions from maps to objects.

We return a conversion when the map element type con convert to all
possible attributes. The conversion itself will fail if the map contains
keys that are not valid attributes of that object type.

[codecov]

Codecov Report

Merging #42 into master will increase coverage by 0.02%.
The diff coverage is 81.25%.

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
+ Coverage   69.57%   69.59%   +0.02%     
==========================================
  Files          78       78              
  Lines        6964     6996      +32     
==========================================
+ Hits         4845     4869      +24     
- Misses       1692     1698       +6     
- Partials      427      429       +2

Impacted Files	Coverage Δ
cty/convert/conversion.go	82.29% <50%> (-1.41%)	⬇️
cty/convert/conversion_collection.go	83.33% <85.71%> (+0.4%)	⬆️
cty/convert/compare_types.go	97.61% <0%> (-2.39%)	⬇️

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[apparentlymart]

Thanks! I rebased this to apply to master without making go.mod invalid, but it's otherwise unchanged.