Resolve ForwardRefs on return annotations

[tlambert03]

This PR also resolves ForwardRefs on return type annotations (#66 only handled ForwardRefs on parameter annotations).

I also changed it so that register_type immediately resolves any ForwardRefs. It's unlikely that someone would be passing a forwardref there, but it will also work now.

[codecov]

Codecov Report

Merging #73 (09ea592) into master (da0383e) will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #73      +/-   ##
==========================================
+ Coverage   87.37%   87.48%   +0.11%     
==========================================
  Files          15       15              
  Lines        1592     1606      +14     
==========================================
+ Hits         1391     1405      +14     
  Misses        201      201              

Impacted Files	Coverage Δ
magicgui/function_gui.py	91.66% <100.00%> (-0.07%)	⬇️
magicgui/type_map.py	94.92% <100.00%> (+0.19%)	⬆️
magicgui/widgets/_bases.py	89.60% <100.00%> (+0.22%)	⬆️

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[sofroniewn]

Just confirmed this now works as expected and fixes napari/napari#2028 (comment). Thanks!!