Implement the StableDeref trait

[SimonSapin]

See https://crates.io/crates/stable_deref_trait

This enables having a complex struct that borrows byte slices from a Mmap, and storing it next to that Mmap to eliminate the lifetime parameter.

[SimonSapin]

@danburkert It looks like Travis-CI is not active on this repo anymore, and I didn’t manage to test Rust 1.13 compat on my machine:

$ cargo +1.13.0 test --verbose
warning: unused manifest key: badges.appveyor.repository
warning: unused manifest key: badges.travis-ci.repository
error: failed to parse lock file at: /home/simon/projects/memmap-rs/Cargo.lock

Caused by:
  invalid serialized PackageId for the key `package.dependencies`
$ rm Cargo.lock; cargo +1.13.0 test --verbose
warning: unused manifest key: badges.appveyor.repository
warning: unused manifest key: badges.travis-ci.repository
    Updating registry `https://github.com/rust-lang/crates.io-index`
error: failed to parse registry's information for: rand

Caused by:
  Feature `std_rng` depends on `rand_hc` which is not an optional dependency.
Consider adding `optional = true` to the dependency

Did you need to pin an older version of rand on CI, for older toolchains?