convert: fix panic when converting nested objects

[mildwonkey]

When converting a list of objects, it is necessary to unify the types of
the child elements. Previously Convert would panic if the nested objects had inconsistent element types.

This PR is nearly identical to #47, but handles lists of objects.

[codecov]

Codecov Report

Merging #53 into master will increase coverage by 0.10%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
+ Coverage   70.36%   70.47%   +0.10%     
==========================================
  Files          79       79              
  Lines        7110     7126      +16     
==========================================
+ Hits         5003     5022      +19     
+ Misses       1663     1661       -2     
+ Partials      444      443       -1     

Impacted Files	Coverage Δ
cty/convert/conversion_collection.go	81.54% <71.42%> (-1.17%)	⬇️
cty/convert/compare_types.go	97.61% <0.00%> (-2.39%)	⬇️
cty/convert/mismatch_msg.go	70.87% <0.00%> (+1.94%)	⬆️
cty/function/stdlib/format_fsm.go	95.58% <0.00%> (+3.02%)	⬆️
cty/convert/unify.go	84.48% <0.00%> (+3.44%)	⬆️

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[apparentlymart]

Thanks! This looks good to me.