Remove context-specific lit

[drhagen]

• Behavior on lit now independent of context
• LiteralParser and LiteralStringParser have been combined
• lit is specialized to generate nice text error messages on StringReader
• Removed global mutable state for lit
• GeneralParsers now responds to whitespace and TextParsers inherits this behavior
• default_whitespace moved to context class attribute
• reg now takes Pattern or str
• RegexParser now takes Pattern only

[codecov-commenter]

Codecov Report

Merging #79 (067dac5) into master (82bb315) will not change coverage. The diff coverage is 100.00%.

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@@            Coverage Diff            @@
##            master       #79   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           25        25           
  Lines          826       813   -13     
  Branches       102       105    +3     
=========================================
- Hits           826       813   -13     

Impacted Files	Coverage Δ
src/parsita/metaclasses.py	100.00% <100.00%> (ø)
src/parsita/options.py	100.00% <100.00%> (ø)
src/parsita/parsers/__init__.py	100.00% <100.00%> (ø)
src/parsita/parsers/_base.py	100.00% <100.00%> (ø)
src/parsita/parsers/_literal.py	100.00% <100.00%> (ø)
src/parsita/parsers/_regex.py	100.00% <100.00%> (ø)

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