Abstracting over the parser

[mgree]

• fix PS1 and PS2 expansion
• "The shell shall replace each instance of the character '!' in PS1 with the history file number of the next command to be typed. Escaping the '!' with another '!' (that is, "!!" ) shall place the literal character '!' in the prompt. This volume of POSIX.1-2017 specifies the effects of the variable only for systems supporting the User Portability Utilities option."

functions needed:

• libdash instance

• support for EvalLoop

  • context (e.g., stackmark/parser state)
  • parse_next function

• set_ps1

• set_ps2

• morbig instance Add a way to run other tools than Morbig on our test suite colis-anr/morbig#102

• Oil instance?