add a command-line shell for LangDef queries

[zerbina]

Summary

Add a simple command-line shell to query LangDefs with and execute
their functions and relations. The source language is currently
hardcoded as the language to query.

Details

The shell is intended to aid with language development, by providing a
convenient way to test relations and functions with various input.

For ease of implementation, commands are provided as S-expressions.
Result values are also rendered as S-expressions. Constructors for
built-in meta-language terms (e.g., true, false, tuples, etc.) are
prefixed with a colon to distinguish them from user-defined type
constructors.

In addition, to showing the result of a function/relation application,
the shell also shows the interpreter-generated trace of which
relations.

Additional Changes

• move the line-buffered stream and S-expression parsing implementations
  from the repl to the new sexpstreams module (queryshell also
  needs both)
• parse colon-prefixed symbols as S-expression symbols in the parse
  iterator, instead of reporting an error
• collect the used rules in traces generated by the interpreter

[zerbina]

A showcase of how the output looks like:

> (default C) (set c)
Got: (:rec (symbols (:map)) (ret (VoidTy)))
> (apply types $c (Call (Ident "+") (Call (Ident "+") (IntVal 1) (IntVal 2)) (IntVal 4)))
- types (:rec (symbols (:map)) (ret (VoidTy))) (Call (Ident "+") (Call (Ident "+") (IntVal 1) (IntVal 2)) (IntVal 4)) (IntTy) | S-builtin-plus
  - mtypes (:rec (symbols (:map)) (ret (VoidTy))) (Call (Ident "+") (IntVal 1) (IntVal 2)) (IntTy) | 
    - types (:rec (symbols (:map)) (ret (VoidTy))) (Call (Ident "+") (IntVal 1) (IntVal 2)) (IntTy) | S-builtin-plus
      - mtypes (:rec (symbols (:map)) (ret (VoidTy))) (IntVal 1) (IntTy) | 
        - types (:rec (symbols (:map)) (ret (VoidTy))) (IntVal 1) (IntTy) | S-int
      - mtypes (:rec (symbols (:map)) (ret (VoidTy))) (IntVal 2) (IntTy) | 
        - types (:rec (symbols (:map)) (ret (VoidTy))) (IntVal 2) (IntTy) | S-int
  - mtypes (:rec (symbols (:map)) (ret (VoidTy))) (IntVal 4) (IntTy) | 
    - types (:rec (symbols (:map)) (ret (VoidTy))) (IntVal 4) (IntTy) | S-int
Got: (IntTy)

It's a bit noisy due to the S-expression-based rendering of record/tuple/set/map values.

Possible future improvements:

• a rendering mode where record/tuple/set/map/values are formatted the same way they're written in the macro DSL (e.g., records as {symbols: {}, ...})
• tracking the lineage of values and omitting identical ones from the trace
• user-provided custom formatting for relations (e.g., so that types a b c shows as a |- b : c)