feat: implement fiber primitives with create, run, yield and caller tracking

Add core fiber functionality including Fiber.create, Fiber.run, Fiber.yield
primitives with value passing between fibers. Introduce PRIM_RUN_FIBER result
type for interpreter loop, add caller field to ObjFiber struct for yield
resumption, and update GC marking to traverse fiber caller chains. Modify
wrenNewFiber to accept a function/closure argument and initialize the first
call frame. Add debug stack printing with fiber pointer. Include test suite
covering fiber creation, run, yield, value passing, type checking, and
caller resumption.

test/fiber/resume_caller.wren

@@ -0,0 +1,19 @@
+var b = Fiber.create(fn {
+  IO.print("fiber b")
+})
+
+var a = Fiber.create(fn {
+  IO.print("begin fiber a")
+  b.run
+  IO.print("end fiber a")
+})
+
+IO.print("begin main")
+a.run
+IO.print("end main")
+
+// expect: begin main
+// expect: begin fiber a
+// expect: fiber b
+// expect: end fiber a
+// expect: end main

test/fiber/run.wren

@@ -0,0 +1,12 @@
+var fiber = Fiber.create(fn {
+  IO.print("fiber")
+})
+
+IO.print("before") // expect: before
+fiber.run          // expect: fiber
+IO.print("after")  // expect: after
+
+// TODO: Test handles error if fiber tries to run itself.
+// TODO: Test create is passed right argument type.
+// TODO: Test closing over stuff in fiber function.
+// TODO: Test running a finished fiber.

test/fiber/run_return_implicit_null.wren

@@ -0,0 +1,6 @@
+var fiber = Fiber.create(fn {
+  IO.print("fiber")
+})
+
+var result = fiber.run // expect: fiber
+IO.print(result)       // expect: null

test/fiber/run_return_value.wren

@@ -0,0 +1,7 @@
+var fiber = Fiber.create(fn {
+  IO.print("fiber")
+  return "result"
+})
+
+var result = fiber.run // expect: fiber
+IO.print(result)       // expect: result

test/fiber/run_with_value.wren

@@ -0,0 +1,9 @@
+var fiber = Fiber.create(fn {
+  IO.print("fiber")
+})
+
+// The first value passed to the fiber is ignored, since there's no yield call
+// to return it.
+IO.print("before")   // expect: before
+fiber.run("ignored") // expect: fiber
+IO.print("after")    // expect: after

test/fiber/type.wren

@@ -0,0 +1,5 @@
+var fiber = Fiber.create(fn null)
+IO.print(fiber is Fiber)      // expect: true
+IO.print(fiber is Object)     // expect: true
+IO.print(fiber is Bool)       // expect: false
+IO.print(fiber.type == Fiber) // expect: true

test/fiber/yield.wren

@@ -0,0 +1,14 @@
+var fiber = Fiber.create(fn {
+  IO.print("fiber 1")
+  Fiber.yield
+  IO.print("fiber 2")
+  Fiber.yield
+  IO.print("fiber 3")
+})
+
+var result = fiber.run // expect: fiber 1
+IO.print("main 1") // expect: main 1
+result = fiber.run // expect: fiber 2
+IO.print("main 2") // expect: main 2
+result = fiber.run // expect: fiber 3
+IO.print("main 3") // expect: main 3

test/fiber/yield_return_value.wren

@@ -0,0 +1,14 @@
+var fiber = Fiber.create(fn {
+  IO.print("fiber 1")
+  var result = Fiber.yield
+  IO.print(result)
+  result = Fiber.yield
+  IO.print(result)
+})
+
+fiber.run // expect: fiber 1
+IO.print("main 1") // expect: main 1
+fiber.run("run 1") // expect: run 1
+IO.print("main 2") // expect: main 2
+fiber.run // expect: null
+IO.print("main 3") // expect: main 3

test/fiber/yield_with_value.wren

@@ -0,0 +1,14 @@
+var fiber = Fiber.create(fn {
+  IO.print("fiber 1")
+  Fiber.yield("yield 1")
+  IO.print("fiber 2")
+  Fiber.yield("yield 2")
+  IO.print("fiber 3")
+})
+
+var result = fiber.run // expect: fiber 1
+IO.print(result) // expect: yield 1
+result = fiber.run // expect: fiber 2
+IO.print(result) // expect: yield 2
+result = fiber.run // expect: fiber 3
+IO.print(result) // expect: null