Add "Interface" to compiled JSON

[elliotchance]

The interface is necessary for later when I removed the PackageFunc.
Also, allowed me to do some cleanup.

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This change is 

[codecov-io]

Codecov Report

Merging #119 (25425ed) into master (b711fc1) will increase coverage by 0.04%.
The diff coverage is 91.94%.

@@            Coverage Diff             @@
##           master     #119      +/-   ##
==========================================
+ Coverage   89.37%   89.42%   +0.04%     
==========================================
  Files         183      182       -1     
  Lines       17340    17440     +100     
==========================================
+ Hits        15498    15595      +97     
- Misses       1703     1706       +3     
  Partials      139      139              

Impacted Files	Coverage Δ
types/type.go	63.63% <0.00%> (-15.61%)	⬇️
vm/interface.go	0.00% <0.00%> (ø)
vm/okc.go	0.00% <0.00%> (ø)
vm/symbol.go	0.00% <ø> (ø)
vm/lib.go	100.00% <100.00%> (ø)

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