Default Backend

[termoshtt]

Related: #102, #103

The default backend features is omitted at version 0.8 when we switch to LAPACKE implementation.
See also how BLAS/LAPACK wrappers select their backend: https://github.com/blas-lapack-rs/blas-lapack-rs.github.io/wiki
Current empty default feature causes an issue as reported in #102, we cannot run cargo test without specifying the backend. The merit of empty default is that we can avoid duplicated dependencies.

[termoshtt]

The [patch] feature introduced in Rust 2018 may help this problem https://doc.rust-lang.org/edition-guide/rust-2018/cargo-and-crates-io/replacing-dependencies-with-patch.html