Implement vim mode

[jasone]

Hemlock syntax is substantially similar to that of OCaml, but there are differences worth highlighting, e.g.:

• Hemlock's keywords don't perfectly overlap.
• Hemlock has different builtin types (codepoint instead of char, usize, additional integer types).
• Hemlock omits objects and polymorphic variants.
• Hemlock has mutation sigils (!, $, and ?).

[jasone]

I already have a kakoune mode for Hemlock, and although it's very basic, I'm finding it adequate for use during bootstrapping. We will codevelop a language server (see #48) during compiler implementation, which subsumes the less ambitious notion of a vim mode.

[cevans87]

I need highlighting from Vim now. It looks like it'll be a fairly mechanical transformation of the Kakoune highlighter spec.

[jasone]

FYI I have some incremental fixes/improvements in my working tree right now.