Allow underscore to indicate an unused value

[Feliix42]

In Rust (and in Haskell as well?) you write something like

let (foo, _) = fancy_func();

to denote the deconstruction of a tuple while only retaining the first element. We might want to introduce this functionality some time in the future, too (doesn't need to be implemented right away).

[sertel]

I wonder why this would currently not work because technically "_" is just a variable name that is just not used elsewhere in the code. Do you see parse errors when doing this?

[Feliix42]

Yep, this is what I get:

ohuac: lexical error at line 7, column 18
CallStack (from HasCallStack):
  error, called at .stack-work/dist/x86_64-osx/Cabal-2.0.1.0/build/Ohua/Compat/Clike/Parser.hs:1029:22 in alang-clike-parser-0.1.0.0-C1uIemENDrx9pCAqtHgztK:Ohua.Compat.Clike.Parser

Line 7, column 18 is _.