Fixed _setval! to work with vector-indices

[torfjelde]

At the moment _setval!(varinfo, chain) will fail because chain does not support element-wise string-indexing, e.g. "y[1]", but chain does support Symbol("y[1]"). Therefore I've just changed it to use symbol- instead of string-indexing.

[codecov]

Codecov Report

Merging #59 into master will not change coverage by %.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #59   +/-   ##
=======================================
  Coverage   80.13%   80.13%           
=======================================
  Files          13       13           
  Lines         901      901           
=======================================
  Hits          722      722           
  Misses        179      179           

Impacted Files	Coverage Δ
src/prob_macro.jl	92.45% <100.00%> (ø)

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