Add stateless to function definitions and remove stateful token

[JSAbrahams]

Relevant issues

Implement #83

Summary

• [x] Add stateless to function definitions. If we encounter these after a def, we start parsing a function definition. The default is stateful.
• [x] Add stateless to top level file definitions. Default should be stateful.
• [x] Re-add class token.
• [x] Remove stateful token.

Added Tests

• [x] Rewrite current tests to check that stateful is the default
• [x] Add tests for stateless

Additional Context

...

[codecov[bot]]

Codecov Report

Merging #85 into master will increase coverage by 0.09%. The diff coverage is 86.5%.

@@            Coverage Diff             @@
##           master      #85      +/-   ##
==========================================
+ Coverage   83.35%   83.44%   +0.09%     
==========================================
  Files          63       63              
  Lines        4085     4108      +23     
==========================================
+ Hits         3405     3428      +23     
  Misses        680      680

Impacted Files	Coverage Δ
src/lexer/mod.rs	96.93% <100%> (ø)	:arrow_up:
src/lexer/token.rs	88% <100%> (ø)	:arrow_up:
tests/desugar/definition.rs	87.25% <100%> (ø)	:arrow_up:
src/desugar/definition.rs	85% <100%> (ø)	:arrow_up:
src/parser/file.rs	86.3% <100%> (-0.19%)	:arrow_down:
src/parser/block.rs	96.66% <100%> (-0.31%)	:arrow_down:
src/parser/definition.rs	86.45% <66.66%> (+0.64%)	:arrow_up:
src/desugar/node.rs	68.07% <77.77%> (-0.15%)	:arrow_down:
tests/parser/valid/expression_and_statement.rs	84.66% <85.71%> (+0.05%)	:arrow_up:
src/parser/ast.rs	25% <85.71%> (+0.71%)	:arrow_up:
... and 1 more

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