Add position information to statements, expressions, and declarations.

[patrickt]

Position information includes filename, line, column, and offset. Pretty straightforward.

This is a backwards-compatible change, and will be tagged as 0.2.2.

[codecov-io]

Codecov Report

Merging #43 into master will increase coverage by 2.31%.
The diff coverage is 50.7%.

@@            Coverage Diff             @@
##           master      #43      +/-   ##
==========================================
+ Coverage   58.17%   60.49%   +2.31%     
==========================================
  Files           1        1              
  Lines         593      653      +60     
==========================================
+ Hits          345      395      +50     
- Misses        216      226      +10     
  Partials       32       32

Impacted Files	Coverage Δ
goblin.go	60.49% <50.7%> (+2.31%)	✅

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