Issue 329: Prevent name conflicts between literals and field names

[treiher]

Closes #329

[codecov]

Codecov Report

Merging #333 into develop will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #333   +/-   ##
========================================
  Coverage    97.95%   97.95%           
========================================
  Files           25       25           
  Lines         4447     4447           
  Branches       746      746           
========================================
  Hits          4356     4356           
  Misses          54       54           
  Partials        37       37           

Impacted Files	Coverage Δ
rflx/generator/common.py	97.60% <ø> (ø)
rflx/generator/core.py	98.65% <100.00%> (ø)

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