Add unused variable check

[eliaslfox]

Add warnings for unused variables:

(coalton-toplevel
  (define (f x) 5))

; caught COMMON-LISP:STYLE-WARNING:
;   warn: Unused variable
;     --> /private/var/tmp/slimedunOe0:2:13
;      |
;    2 |    (define (f x) 5))
;      |               ^ variable defined here

Variables can be marked as intentionally used by prefixing them with _. If an intentionally ignored variable is used, then a warning is generated:

(coalton-toplevel
  (define (f _x) _x))

;   warn: Use of ignored variable
;     --> /private/var/tmp/slimeQxQzkn:2:17
;      |
;    2 |    (define (f _x) _x))
;      |               -- definition here
;      |                   ^^ first usage here

Fixes #12

[stylewarning]

FWIW, I've enjoyed the distinction of IGNORE and IGNORABLE in CL declarations, mostly for macro-generated code. But I do like the _x convention for most practical uses of ignoring a variable.

[gefjon]

But I do like the _x convention for most practical uses of ignoring a variable.

I'm a fan of Rust's using _ (the underscore with nothing after) as equivalent to CL's ignore, and _foo (an underscore with any valid identifier after) as ignorable.

[eliaslfox]

I'm a fan of Rust's using _ (the underscore with nothing after) as equivalent to CL's ignore, and _foo (an underscore with any valid identifier after) as ignorable.

I like this. I'll update this pr to make _foo equivalent to ignorable. Underscore bindings will be part of argument pattern matching (now tracked at #865).