Replace purely hashmap based keymaps with function-based mappings (basically a parser-combinator style API?)

[elkowar]

Currently, input-mapper expresses keymaps as nested hashmaps.
This has a few very key limitations:

• It doesn't allow us to match on arbitrary keys by some property (as evidenced by the fact that we already have a special cased fallback "any_printable" case)
• It means that for numbers and anything like that, we need to basically store all keymaps 10x
• the api is kinda eh

Optimally, every keymap would just consist of a function that takes an input, does whatever to it, and returns a new submap (-> another function) or the leaf value.
This would simplify a lot of the more complex logic here, and should be possible to express relatively cleanly through a parser-combinator style API. However, it's not yet clear how we'd do descriptions with that...

This is a followup to the implementation of #86 done in #100