Rewrite REPL mode

[MasonProtter]

Something that should be thought about is creating a REPL mode for Rewrite so that one doesn't have to stick @term everywhere and to make certain other interfaces nicer. I have a framework for making such a mode here: https://github.com/MasonProtter/ReplMaker.jl so maybe I'll try my hand at cooking up at Rewrite repl mode.

[HarrisonGrodin]

Having a custom REPL mode would be fantastic! I meant to add an issue for it. If you put together a PR, that'd be incredible.

A few features that we should probably include:

• Normalization under the default rules (e.g. normalize(@term diff(sin(x), x)))
• Normalization under a specific set of rules (e.g. normalize(@term(diff(sin(x), x)), :CALCULUS))
• Normalization under a custom set of rules (e.g. normalize(@term(x + 0), @term RULES [x + 0 => x]))
• Completion, given custom axioms and a reduction order

The first is the most important for the short-term, since set_context! could be used in the standard mode to deal with custom rules. The others would be nice to have at some point, but could be split into separate issues.

[MasonProtter]

I'm putting together a basic REPL mode that covers the first point. We can talk strategy for the later points once I submit that.

[HarrisonGrodin]

Closing (see #42).