Declare post-condition result variable as reference if return type is a resource

[turbolent]

Description

Function post-conditions may use the special variable result, which is the value that is returned from the function.

When the return type of a function is a resource type, then declare the type of the special variable result to be a reference type.

This is a port of

• dapperlabs/cadence-internal#4
• a part of dapperlabs/cadence-internal#8

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• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[codecov-commenter]

Codecov Report

Merging #905 (1592c50) into master (932f51f) will decrease coverage by 0.00%.
The diff coverage is 45.45%.

@@            Coverage Diff             @@
##           master     #905      +/-   ##
==========================================
- Coverage   39.99%   39.99%   -0.01%     
==========================================
  Files         254      254              
  Lines       31721    31736      +15     
==========================================
+ Hits        12688    12693       +5     
- Misses      18179    18187       +8     
- Partials      854      856       +2     

Flag	Coverage Δ
unittests	39.99% <45.45%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
runtime/sema/check_function.go	0.00% <0.00%> (ø)
runtime/interpreter/interpreter.go	21.13% <66.66%> (+0.23%)	⬆️

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