Inspector for continuations (and maybe generics)

[alaviss]

Basically tools to inspect a continuation. This includes:

• Local variables
• Result
• Bootstrap
• And more...

This should allow users to look into the insides of a continuation and produce procs that can allow forking (by copying all locals and construct a new continuation), etc.

One of the benefits for CPS is that we can stop producing helpers like () and instead use a generic implementation like:

template `()`[T: Continuation](c: T): untyped =
  when c.hasResult:
    if c.finished:
      c.getResult
    else:
      trampoline c
  else:
    trampoline c

Which would allow us to not care about the scope of which a CPS procedure resides in, opening the possibilities for cps generics, like this for example:

# instantiated body for T = int
proc foo[T](x: T): T {.cpsProc.} =
  {.cpsEnv: EnvType.}:
    type
      EnvType {.cpsBooty: booty.} = ref object of Continuation
        x_0* {.cpsLocal.}: int
        result_0* {.cpsResult.}: int

    proc leg0(c: Continuation): Continuation {.nimcall, cpsLeg: EnvType.} = ...
    proc leg1(c: Continuation): Continuation {.nimcall, cpsLeg: EnvType.} = ...

    proc booty(x: int): EnvType {.nimcall, cpsBooty.} = ...

    # Default impl
    tramp(booty(x)).getResult

proc bar(x: int) {.cps: Continuation.} =
  echo foo(x)

  # Which becomes something like:
  c.tmp = whelp foo(x) # => booty(x)
  c.fn = next
  c.tmp.mom = thisContinuation
  return c.tmp

  proc next(c: Continuation): Continuation =
    let :tmp = c.tmp.getResult
    echo :tmp

  # Of which there is no dependency on `foo` inner scopes except for the initial
  # `foo()` symbol