[WIP] Multi-chain repl support

[jmcardon]

This PR aims to implement #96. To achieve this, we need to do a few things:

• load must be moved from a repl special form to a native. That said: There's no meaningful return value for load, so we may just return anything.
• We need to change the way we do repl tests/inspect values for failure/passing. Currently load allows all repl script values to be inspectable by the repl tests. We may want to instead append them to some sort of logger to make the repl tests work.
• TODO: we want a few natives to set the chain, resume on a target chain, etc.