Replace parser combinators by FastParse

[alonsodomin]

This PR rewrites the whole implementation of the parser using FastParse instead of Scala Parser Combinators. The reasons behind this change are:

• FastParse is faster, specially when parsing a self-recursive AST like the one in cron4s
• Better error messages when parsing fails. Scala Parser Combinators is able to report on the position in which the parser failed but not which was the depest sub-parser that performed the attempt and failed. This feature in FastParse will enable providing with better and more comprehensive error messages.
• Slow release cycle in Scala Parser Combinators, specially noticed whilst waiting for a Scala 2.12 ScalaJS compatible artifacts.

[codecov-io]

Current coverage is 95.35% (diff: 91.30%)

Merging #22 into master will decrease coverage by 1.44%

@@             master        #22   diff @@
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  Files            42         40     -2   
  Lines           499        495     -4   
  Methods         470        470          
  Messages          0          0          
  Branches         29         25     -4   
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- Hits            483        472    -11   
- Misses           16         23     +7   
  Partials          0          0          

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