make parser capable of treating AST instuctions as first-class symbols

[hhefesto]

Original Issue: sfultong/stand-in-language#29

E.G. you should be able to pass AST instructions as arguments, like app (pair zero zero) left

so right now in a .sil file, we could have a line like
test = left {0,0}

left is a keyword, and the parser always expects it to be followed by an argument.

So if I wanted to use it in a function, I'd have to do something like
test = (\f -> f {0,0}) (\x -> left x)

I want to be able to do this instead:
test = (\f -> f {0,0}) left

So we can parse left as a regular function, but it should also generate the same grammar as before when parsing old syntax (e.g. left {0,0})