MEMORY_BUSY during BLE operations

[gfwilliams]

The current bluetooth implementation does a lot of things in IRQs (for instance promise resolution) that allocate variables - Espruino was never meant for that, and you can occasionally hit an issue where Espruino is unable to allocate a variable from an IRQ because it's doing GC in the main thread.

Not such an issue for advertising, but a deal-breaker during a promise.

I've spent a while trying to improve matters quite a lot, but at the end of the day it's almost unavoidable - so I want to move away from allocating JsVars, and to instead use the IO queue that we use for other events (eg BLE UART, Serial, Watches, etc).

[gfwilliams]

This is mostly done now, but we need a way to push to and retrieve data from the IO queue, so we can move:

• write and notify
• connect (it has an address)
• advertising (we currently use a static buffer, but that'll only hold one advertising packet)

[gfwilliams]

Complete as of a month or so ago