Rewrite Lexer & Parser (#1)

This PR rewrites the lexer to:

 - Half the size of tokens from 32B to 16B.
 - Skip whitespace & comments in the lexer, not the parser.
 - Simplify some tokens (e.g a Percentage token is not needed).
 - Move span data from the lexer to the parser.

It also rewrites the parser to:

- Move all individual parsing logic to hdx_ast, colocating it with the
AST nodes
 - Make use of more macros & Derives to massively DRY up parsing logic

The aim is to:

 - Speed up overall parsing
 - Make it easier to develop
 - Make the whole thing more robust.

.editorconfig

@@ -1,6 +1,7 @@
 root = true
 
 [*]
+max_line_length = 120
 end_of_line = lf
 insert_final_newline = true
 charset = utf-8