latex,ast,token: simplify ast and token

[sbinet]

Fixes #10.
Fixes #11.

[codecov-io]

Codecov Report

Merging #13 into master will increase coverage by 0.29%.
The diff coverage is 55.73%.

@@            Coverage Diff             @@
##           master      #13      +/-   ##
==========================================
+ Coverage   96.33%   96.62%   +0.29%     
==========================================
  Files          14       14              
  Lines        9077     9020      -57     
==========================================
- Hits         8744     8716      -28     
+ Misses        309      280      -29     
  Partials       24       24              

Impacted Files	Coverage Δ
token/kind_string.go	8.69% <0.00%> (+2.98%)	⬆️
token/token.go	100.00% <ø> (ø)
ast/ast.go	56.31% <40.00%> (+2.22%)	⬆️
parser.go	77.29% <72.72%> (-2.71%)	⬇️
scanner.go	87.40% <100.00%> (+8.75%)	⬆️

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