Do a bunch of work on the HolyC version

[slendidev]

This PR implements a basic line-by-line REPL. It also updates the HolyC Vim syntax highlight.

Edit: I got bored of waiting so I started to implement the parser. It can parse mathematical expressions, let, return. It also prints out the AST in a nice way.

It doesn't show in the screenshot, but yes, order of operations also works. I decided to not follow the book on generating the AST and making the parser because it would require a bunch of function pointers and ugly syntax. It is still recursive descent tho.

[slendidev]

@vhladko can you please take a look at this PR? I want to get to working on parser and AST.