Stop using `Core.Compiler.return_type`

[oscardssmith]

Base.promte_op is a strictly less sketchy version.

[andyferris]

Hmm, there seems to be an issue with broadcast?

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 57.14286% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 80.51%. Comparing base (27b59aa) to head (b0a1d3d). Report is 2 commits behind head on master.

Files	Patch %	Lines
src/Dictionary.jl	0.00%	2 Missing :warning:
src/map.jl	66.66%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #140      +/-   ##
==========================================
+ Coverage   80.48%   80.51%   +0.03%     
==========================================
  Files          21       21              
  Lines        2357     2356       -1     
==========================================
  Hits         1897     1897              
+ Misses        460      459       -1     

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[oscardssmith]

should this be merged?