Take advantage of the new Executor::step API in cambridge-asm

[SaadiSave]

• Make UI to monitor register and memory contents throughout execution
• Add buttons for stepwise execution and pausing

[SaadiSave]

Add two functions: parse and step
For step, the Executor is not wasm-bindgen compatible, so use once_cell to initialise during parse and then step through it during execution.

The step function should return the Context at every step, so serialize it to JsValue using serde-wasm-bindgen and return. Then, define a Context type in typescript, like this:

type MemEntry = {
    literal: number,
    address: number | undefined
}

type Memory = Map<number, MemEntry>

type Context = {
    cmp: boolean,
    mar: number,
    acc: number,
    ix: number,
    flowOverrideReg: boolean,
    mem: Memory,
    ret: number,
    gprs: number[],
    end: boolean,
}

And cast like this:

let ctx = step() as Context

*To allow serialization, add a conditional derive of Serialize on Context in cambridge-asm