New flag: '--debug-dprint'

[elegios]

This PR (finally!) enables mi to replace dprint with essentially (lam x. print (pprintTy x); flushStdout ()), where pprintTy is an auto-generated function based on the inferred type of the dprint. This is based on generate-pprint.mc. I also fix a bug with generating code for user-defined types. There are some important caveats however:

• When using the old pipeline there's no terribly convenient way of making sure we have the functions we need in scope. If a required function (e.g., int2string or seq2string) isn't in scope, the compiler will insert an unsymbolized variable, which makes the OCaml backend error out later, saying something like "unbound variable nv_int2string. This is the primary reason this feature is behind a flag.
  • This can interact poorly with dead code elimination. For example, when using this in miking-dppl I have to ensure there's a reference somewhere to seq2string, because that function is otherwise not used, even if the file it's defined in (string.mc) is included.
• The compiler should insert something that can run in all cases, even for things that can't reasonably be inspected. For example, polymorphic values will be printed as <poly varName>, and types that don't have support in generate-pprint.mc will print as <missing case>.

As an example, in code I'm working on right now some DPPL terms aren't removed and instead survive to reach the ocaml backend. Adding a dprint in the wildcard case, then compiling the cppl compiler with mi compile --debug-dprint I can run and get this error message:

(Assume_TmAssume {ty = (FloatTypeAst_TyFloat {info = (Info {col1 = 14, col2 = 56, row1 = 57, row2 = 57, filename = "/home/vipa/repositories/miking-dppl/coreppl/models/ssm.mc"})}), info = (Info {col1 = 6, col2 = 12, row1 = 57, row2 = 57, filename = "/home/vipa/repositories/miking-dppl/coreppl/models/ssm.mc"}), dist = (VarAst_TmVar {ty = (UnknownTypeAst_TyUnknown {info = (NoInfo ())}), info = (Info {col1 = 14, col2 = 56, row1 = 57, row2 = 57, filename = "/home/vipa/repositories/miking-dppl/coreppl/models/ssm.mc"}), ident = ("dist", <missing case for Symbol>), frozen = false}), driftKernel = (Some (LamAst_TmLam {ty = (UnknownTypeAst_TyUnknown {info = (NoInfo ())}), info = (Info {col1 = 14, col2 = 56, row1 = 57, row2 = 57, filename = "/home/vipa/repositories/miking-dppl/coreppl/models/ssm.mc"}), ident = ("", <missing case for Symbol>), body = (VarAst_TmVar {ty = (UnknownTypeAst_TyUnknown {info = (NoInfo ())}), info = (Info {col1 = 14, col2 = 56, row1 = 57, row2 = 57, filename = "/home/vipa/repositories/miking-dppl/coreppl/models/ssm.mc"}), ident = ("dist", <missing case for Symbol>), frozen = false}), tyAnnot = (UnknownTypeAst_TyUnknown {info = (NoInfo ())}), tyParam = (UnknownTypeAst_TyUnknown {info = (NoInfo ())})}))})ERROR: infoTm not implemented for OCaml terms!

...or with some extra newlines for readability:

(Assume_TmAssume
  { ty = (FloatTypeAst_TyFloat {info = (Info {col1 = 14, col2 = 56, row1 = 57, row2 = 57, filename = "/home/vipa/repositories/miking-dppl/coreppl/models/ssm.mc"})})
  , info = (Info {col1 = 6, col2 = 12, row1 = 57, row2 = 57, filename = "/home/vipa/repositories/miking-dppl/coreppl/models/ssm.mc"})
  , dist = (VarAst_TmVar
    { ty = (UnknownTypeAst_TyUnknown {info = (NoInfo ())})
    , info = (Info {col1 = 14, col2 = 56, row1 = 57, row2 = 57, filename = "/home/vipa/repositories/miking-dppl/coreppl/models/ssm.mc"})
    , ident = ("dist", <missing case for Symbol>)
    , frozen = false})
  , driftKernel = (Some (LamAst_TmLam
    { ty = (UnknownTypeAst_TyUnknown {info = (NoInfo ())})
    , info = (Info {col1 = 14, col2 = 56, row1 = 57, row2 = 57, filename = "/home/vipa/repositories/miking-dppl/coreppl/models/ssm.mc"})
    , ident = ("", <missing case for Symbol>)
    , body = (VarAst_TmVar {ty = (UnknownTypeAst_TyUnknown {info = (NoInfo ())}), info = (Info {col1 = 14, col2 = 56, row1 = 57, row2 = 57, filename = "/home/vipa/repositories/miking-dppl/coreppl/models/ssm.mc"}), ident = ("dist", <missing case for Symbol>), frozen = false})
    , tyAnnot = (UnknownTypeAst_TyUnknown {info = (NoInfo ())})
    , tyParam = (UnknownTypeAst_TyUnknown {info = (NoInfo ())})
    }))
  })
ERROR: infoTm not implemented for OCaml terms!

Note <missing case for Symbol>, that's because Symbol is a TyCon even though it's a builtin, and I haven't made the extra effort of adding something that looks for that.