A Roblox-like scheduler for mlua

• Tokio support + minimal CPU usage
• Correctness and performance are king
• Simple with simple primitives for getting results out.
• Properly working coroutine.yield and coroutine.resume functions that produce equivalent (mostly) results in Roblox's own Luau + Task Scheduling code
• Both mlua non-send and send features supported (thanks to rust async wizardry).
• Custom async function handling that works with Lua's coroutine design paradigm without breaking on edge-cases.
• Simple callback API for controlling the scheduler (SchedulerFeedback trait)

• Thread arguments are not preloaded to the stack due to mlua limitations. This is normally not a problem but may cause side-effects. For example, see the below.

local task = task or require("@lune/task")
local thread = task.delay(1, function(foo)
    print(foo)
    print(coroutine.yield())
    print("c")
    print(coroutine.yield())
    print("e")
end, "b")

task.spawn(thread, "a")

task.delay(2, function()
    coroutine.resume(thread, "d")
end)

mlua_scheduler has two scheduler versions. v1 (the original scheduler which used busy poll loop/spinloop that ran every tick to run through tasks) and v2 which is a new scheduler design based on tokio select and delayqueue/tokio channels without any busy polling. Here are the differences:

mluau_scheduler no longer supports the v1 scheduler. The v2 scheduler is the only one supported and is the default.

• V2 scheduler is based on the Tokio runtime and uses its async features.
• V2 scheduler is more efficient and does not use busy polling.
• V2 scheduler is more robust and can handle more complex tasks with minimal performance loss to the v1 scheduler.