Wasm Kernel: feature flagging host capabilities + test kernels

Goal

Allow choice of kernel out of a group of implemented code, where it's known which host capabilities each kernel depends on.

A choice of kernels should be available which use the host capabilities in a stack of:

• computation only
• panic without hook
• panic after N ticks with hook (uses shutdown)
• Debug output
• Echo Input -> Debug
• Echo Input -> Output

Implementation

Each host capability should live under it's own feature flag - and each individual kernel should have it's own feature flag which in turn includes those host capabilities required.