add GetValueSymbols and GetTypeSymbols

[issadarkthing]

I've been using anko for my music player and I wanted to implement auto completion. By adding GetValueSymbols and GetTypeSymbols, I will be able to easily fetch symbols of the current scope. This will also help others with the same use case.

[codecov-io]

Codecov Report

Merging #347 (2607b99) into master (0d30f07) will decrease coverage by 0.75%. The diff coverage is 50.00%.

:exclamation: Current head 2607b99 differs from pull request most recent head e4c5376. Consider uploading reports for the commit e4c5376 to get more accurate results

@@            Coverage Diff             @@
##           master     #347      +/-   ##
==========================================
- Coverage   88.40%   87.64%   -0.76%     
==========================================
  Files          15       15              
  Lines        2682     2550     -132     
==========================================
- Hits         2371     2235     -136     
- Misses        201      205       +4     
  Partials      110      110              

Impacted Files	Coverage Δ
env/envValues.go	93.75% <50.00%> (-6.25%)	:arrow_down:
ast/astutil/walk.go	42.85% <0.00%> (-3.26%)	:arrow_down:
vm/vmToX.go	84.69% <0.00%> (-1.29%)	:arrow_down:
vm/vmExprFunction.go	75.76% <0.00%> (-1.16%)	:arrow_down:
anko.go	67.44% <0.00%> (-1.04%)	:arrow_down:
vm/vm.go	91.26% <0.00%> (-0.67%)	:arrow_down:
vm/vmConvertToX.go	93.33% <0.00%> (-0.55%)	:arrow_down:
vm/vmStmt.go	96.01% <0.00%> (-0.19%)	:arrow_down:
vm/vmExpr.go	96.20% <0.00%> (-0.09%)	:arrow_down:
vm/vmLetExpr.go	85.71% <0.00%> (-0.06%)	:arrow_down:
... and 6 more

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