Improve debugability of generated JS code

[LPTK]

• Print runtime error stack traces
• Generate sanity checks as part of the JS code (with a flag to disable that when more performance is needed):
  • check argument numbers
  • check result of field selections is not undefined
• Add a flag to instrument generated JS so we can see what the program is doing in terms of the usual indented traces with function arguments and results
  Add a Debug.mls file to hkmc2/shared/src/test/mlscript-compile, to be imported implicitly, where the tracing logic is implemented in MLscript.
• Add fun by-name parameters: for example, fun foo(fun thunk) = if ... then thunk else ()

For points 2 and 3, don't modify the Lowering.scala code directly: instead, make a trait to be mixed in that overrides selected functions from it. Modularize Lowering.scala as needed.