Fix fuzz warnings and deny warnings by default in build system

[GuillaumeGomez]

Fixes #712.

Fix the following warnings:

warning: unused variable: `curr`
  --> src/fuzz/reduce.rs:42:13
   |
42 |     let mut curr = 0;
   |             ^^^^ help: if this is intentional, prefix it with an underscore: `_curr`
   |
   = note: `#[warn(unused_variables)]` on by default

warning: variable does not need to be mutable
  --> src/fuzz/reduce.rs:42:9
   |
42 |     let mut curr = 0;
   |         ----^^^^
   |         |
   |         help: remove this `mut`
   |
   = note: `#[warn(unused_mut)]` on by default

warning: unused `Result` that must be used
   --> src/fuzz/reduce.rs:431:9
    |
431 |         out.write_all(line.as_bytes());
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: this `Result` may be an `Err` variant, which should be handled
    = note: `#[warn(unused_must_use)]` on by default
help: use `let _ = ...` to ignore the resulting value
    |
431 |         let _ = out.write_all(line.as_bytes());
    |         +++++++

warning: `y` (bin "y") generated 3 warnings (run `cargo fix --bin "y"` to apply 1 suggestion)

[antoyo]

LGTM. I'll let @FractalFir approve just to make sure he didn't have other plans for this variable.

[FractalFir]

This looks correct. curr was just a remnant from an older version of this pass, which tried to apply this reduction on a per-function basis.

Now, it tries to apply it on sections of the file.

[antoyo]

Thanks for doing this!

[antoyo]

Thanks for your contribution!