Stack safe recursion

[CAG2Mark]

Uses effect handlers to implement stack safe recursion.

Depends on #257, which should be merged first.

TODO:

• Write more tests and check for bugs
• __Return is currently leaked, which should be fixed
• Don't rewrite trivial functions

[LPTK]

Notes from today's meeting

Example that does not work:

:stackSafe 100
:handler
fun foo(f) = print(f(f)) // or just `f(f)`
foo(foo)
//│ FAILURE: Unexpected runtime error
//│ ═══[RUNTIME ERROR] RangeError: Maximum call stack size exceeded
//│     at foo (REPL11:1:60)
//│     at foo (REPL11:1:3053)
//│     at foo (REPL11:1:3053)
//│     at foo (REPL11:1:3053)
//│     at foo (REPL11:1:3053)
//│     at foo (REPL11:1:3053)
//│     at foo (REPL11:1:3053)
//│     at foo (REPL11:1:3053)
//│     at foo (REPL11:1:3053)
//│     at foo (REPL11:1:3053)

To preserve tail calls, we want to update the stack depth global variable on function calls:

let tmp = __stackDepth
set __stackDepth = tmp + 1
tailCall()

let tmp = __stackDepth
set __stackDepth = tmp + 1
nonTailCall()
__stackDepth = tmp

However, the __stackDepth = tmp will interact poorly with effect resumptions. How to deal with this?