Validation-related Performance Optimizations

[hcpchris]

• Made the use of Esprima optional when calling .to_js_literal() (and disabled by default), leading to a 50 - 90% performance improvement. Closes to_js_literal method is taking excessively long time #51 .
• Refactored some of the data validation / data conversion functionality in the .to_json() methods, leading to a 30 - 50% performance improvement.

[codecov]

Codecov Report

Patch coverage: 46.95% and project coverage change: -0.16% ⚠️

Comparison is base (c8217a9) 89.32% compared to head (52e3795) 89.17%.
Report is 1 commits behind head on v.1.4-rc-branch.

Additional details and impacted files

@@                 Coverage Diff                 @@
##           v.1.4-rc-branch     #102      +/-   ##
===================================================
- Coverage            89.32%   89.17%   -0.16%     
===================================================
  Files                  207      207              
  Lines                29182    29218      +36     
  Branches              2102     2116      +14     
===================================================
- Hits                 26067    26055      -12     
- Misses                2328     2377      +49     
+ Partials               787      786       -1     

Files Changed	Coverage Δ
...harts_core/utility_classes/javascript_functions.py	91.26% <ø> (ø)
highcharts_core/js_literal_functions.py	58.91% <37.75%> (-9.95%)	⬇️
highcharts_core/chart.py	46.08% <100.00%> (ø)
highcharts_core/global_options/shared_options.py	70.27% <100.00%> (ø)
highcharts_core/metaclasses.py	62.02% <100.00%> (ø)
highcharts_core/options/series/data/base.py	82.08% <100.00%> (ø)
highcharts_core/options/series/data/collections.py	60.68% <100.00%> (ø)

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