CPU fuel and memory limit

[amitu]

When a request is getting handled, we should allocate a configurable but finite "fuel" and memory, and during parsing etc, we keep track of how much fuel is consumed, eg every byte read, is one unit, every cpu instruction another fuel and so on.

Read fuel handling in wasmtime: https://docs.rs/wasmtime/latest/wasmtime/struct.Config.html#method.consume_fuel for inspiration.

Also check their memory management primitive: https://docs.rs/wasmtime/latest/wasmtime/trait.MemoryCreator.html

[amitu]

There is also a possibility that fastn itself uses wasmtime internally, so multiple versions of fastn can be used in the same process, and so we get the fuel/memory limit stuff from wasmtime. This will come at a cost, programs would run almost twice as slow as native.