Add function yielding and resuming

Sometimes, a builtin function may need to call out to another function
(user-defined or otherwise). Previously, we were just calling the
function and popping the stack frame, leaving no room for the new
function to be called. This introduces a `FunctionResult` and
`FunctionState` that get passed between these builtin functions. A
builtin function will receive a FunctionState that tells it whether it
is currently beginning or being resumed and can act accordingly. A
builtin function will in turn return a FunctionResult, which can either
be to return and push a value to the stack, return without pushing a
value (value is already on top of the stack), or yield execution back to
the VM (implying that a new stack frame has been pushed with a new
function to execute).

Having to call a new function and resume is a bit unwieldy and
un-ergonomic, and making a macro to help write these would be nice, but
it looks like a procedural macro may be required to really enable this.
For now, we will write these yields by hand and once it becomes truly
too much, we can start looking at writing a macro library to handle this
case.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/disassemble.rs

@@ -99,6 +99,11 @@ fn disassemble_chunk(chunk: &Chunk, constants: &Vec<ObjP>, globals: &Vec<String>
                 arg = format!("{depth}");
                 info = format!("slot {slot} (name unknown)");
             }
+            Op::Nop => {
+                op_str = "NOP";
+                arg = String::new();
+                info = String::new();
+            }
             Op::Halt => {
                 op_str = "HALT";
                 arg = String::new();