NOTWORKING: use cfg bench instead of feature unstable

[RCasatta]

attempt to fix #58

:~/git/blocks_iterator(cfg_bench)$ RUSTFLAGS='--cfg=bench' cargo +nightly bench
   Compiling blocks_iterator v1.0.0 (/home/casatta/git/blocks_iterator)
error[E0432]: unresolved import `sha2`
   --> src/lib.rs:287:9
    |
287 |     use sha2::{Digest, Sha256};
    |         ^^^^ use of undeclared crate or module `sha2`

error: unused imports: `DB`, `Options`, `WriteBatch`
   --> src/utxo/db.rs:184:19
    |
184 |     use rocksdb::{Options, WriteBatch, DB};
    |                   ^^^^^^^  ^^^^^^^^^^  ^^
    |
note: the lint level is defined here
   --> src/lib.rs:18:9
    |
18  | #![deny(unused_imports)]
    |         ^^^^^^^^^^^^^^

error: unused import: `test::Bencher`
   --> src/utxo/db.rs:185:9
    |
185 |     use test::Bencher;
    |         ^^^^^^^^^^^^^

error: unused imports: `HashEngine`, `Hash`, `sha256`
   --> src/lib.rs:284:34
    |
284 |     use crate::bitcoin::hashes::{sha256, Hash, HashEngine};
    |                                  ^^^^^^  ^^^^  ^^^^^^^^^^

error: unused import: `crate::bitcoin::OutPoint`
   --> src/lib.rs:285:9
    |
285 |     use crate::bitcoin::OutPoint;
    |         ^^^^^^^^^^^^^^^^^^^^^^^^

error: unused import: `bitcoin::hashes::sha256::Midstate`
   --> src/lib.rs:286:9
    |
286 |     use bitcoin::hashes::sha256::Midstate;
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: unused imports: `Bencher`, `black_box`
   --> src/lib.rs:288:16
    |
288 |     use test::{black_box, Bencher};
    |                ^^^^^^^^^  ^^^^^^^

For more information about this error, try `rustc --explain E0432`.