Fix parsing of negative fractional fixed-point strings

[mikeylemmon]

Closes #934

Description

A bug in parseFixedPoint was causing the function to return positive values for strings like "-0.1" because it was only checking if the integer component is less than zero. This PR fixes the bug by ensuring the return value is negative if the string begins with '-'.

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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 #935 (040ad7d) into master (2d95036) will decrease coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #935      +/-   ##
==========================================
- Coverage   74.19%   74.05%   -0.14%     
==========================================
  Files         267      269       +2     
  Lines       32993    33074      +81     
==========================================
+ Hits        24479    24494      +15     
- Misses       7386     7452      +66     
  Partials     1128     1128              

Flag	Coverage Δ
unittests	74.05% <100.00%> (-0.14%)	⬇️

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

Impacted Files	Coverage Δ
fixedpoint/parse.go	93.42% <100.00%> (-0.09%)	⬇️
runtime/ast/transaction_declaration.go	61.11% <0.00%> (-12.23%)	⬇️
runtime/ast/statement.go	72.89% <0.00%> (-8.87%)	⬇️
runtime/ast/variable_declaration.go	70.37% <0.00%> (-8.80%)	⬇️
runtime/ast/block.go	87.50% <0.00%> (-7.96%)	⬇️
runtime/ast/interface.go	81.25% <0.00%> (-5.42%)	⬇️
runtime/ast/pragma.go	68.75% <0.00%> (-4.59%)	⬇️
runtime/ast/expression.go	66.74% <0.00%> (-3.33%)	⬇️
runtime/ast/function_declaration.go	63.26% <0.00%> (-2.70%)	⬇️
runtime/ast/program.go	71.92% <0.00%> (-1.29%)	⬇️
... and 11 more

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[Kay-Zee]

Repro for the problem. https://play.golang.org/p/URyXIX-HTU_S

[turbolent]

@mikeylemmon Thank you for reporting this bug and also fixing it! 👍

I'll add some tests.