OSH Parser

These facts are useful for the parsing contest.

• 15 lexer modes (lexical state)
• 233 IDs (token types / node types) in 23 kinds (core/id_kind_test.py shows this)
• 3 recursive descent parsers (command, word, [[)
• 1 Pratt parser (arithmetic)
• [ fallback reuses osh/bool_parser
• TODO: modify asdl.py to show these stats?
  • X product types
  • X sum types with X alternatives

• what CPython opcodes does it use?
• how many lines of code does it use in CPython? (Compare with execution.)
• What is the distribution of ASDL string and array lengths per node type?
  • note: there are several uses of string, not just token. Is this a good or bad optimization?

• Brace detection -- this is a separate metaprogramming pass (doesn't depend on input). This is a recursive parser, although it operates entirely on token types and not chars/strings?
• core/glob_.py -- escape and unescape
• TODO: regex escape, for passing to regcomp()

• core/word_eval.py -- after evaluating VarOp arguments, we compile globs to Python regexes, e.g. for ${x%foo*}
• IFS splitting (this is quite slow and needs to be sped up!)
• core/args.py -- this is not a recursive parser
• echo -e -- backslash escapes (and printf if it turns out we need it as a builtin)
• read without -r -- backslash escapes are parsed

• Polymorphism:
  • FileLineReader, StringLineReader, VirtualLineReader for here docs.
  • BoolParser can take test_builtin._StringWordEmitter or WordParser