REPL-like experience

[jemc]

Many users of languages like Ruby and Elixir are accustomed to having a REPL tool, where they can make use of an interactive prompt to explore available methods and try out code snippets.

We'd like to support something similar to this in Savi, though perhaps not a full-on REPL, because it is more difficult to do a full REPL in a strongly typed compiled language, than it is to do in a dynamic, interpreted language like Ruby. Crystal has no REPL for this reason. Rust has no official REPL, but there are some user projects aiming to bridge this gap.

In this ticket we'd like to gather relevant use cases for the REPL experience so we can decide on a feature set that gives users most of what they need/want, but without becoming prohibitively complex to implement or maintain. We should be able to meet basic needs using some caching / incremental compilation tricks we already have in the compiler, but we first need to gather features/requirements and make a plan.