vine: support (mutually) recursive `let fn`s

[tjjfvi]

Currently, let fns are only in scope after the statement, so they cannot be recursive. Instead, it should be that, all let fns in a consecutive sequence of let fn statements are in scope from the first such statement.

For example, in the following code:

/* 1 */ let x = ...;
/* 2 */
/* 3 */ let fn foo(...) { ... }
/* 4 */ let fn bar(...) { ... }
/* 5 */ let fn baz(...) { ... }
/* 6 */
/* 7 */ let y = ...;
/* 8 */
/* 9 */ let fn qux(...) { ... }

foo, bar, and baz would be in scope for lines 3 onward, and qux would be in scope for lines 9 onward.

This would make let fns arguably the most primitive form of control flow in Vine; they correspond nearly 1-to-1 to transfers in the SFG.

[enricozb]

Drive by comment, IMO this style of whitespace sensitivity (see edit) is pretty unintuitive instead of a clearer block construct like OCaml's

let rec f = ...;
and g = ...;
and h = ...;

Edit: reading more closely, what you're suggesting isn't whitespace sensitivity.

[tjjfvi]

Oh, yeah, not whitespace sensitivity. Just that intermediate statements break the block. Which is necessary to ensure variables can't be used before they're declared. Still slightly magic, though.

Could consider something like

let
  fn foo(...) { ... },
  fn bar(...) { ... },
  fn baz(...) { ... },
;

But I'm not sure I like the look of it.

I'm open to other suggestions.