add #![feature(i128_type)] to the crate attributes to enable
build failed with command: `cargo build --release` on macos, the version of rust is:

```text
$ rustup show                                                                                                                                                             *[master]
Default host: x86_64-apple-darwin

installed toolchains
--------------------

stable-x86_64-apple-darwin
nightly-x86_64-apple-darwin (default)

active toolchain
----------------

nightly-x86_64-apple-darwin (default)
rustc 1.25.0-nightly (ee220daca 2018-01-07)
```

```text
Compiling config v0.8.0
   Compiling bandit v0.11.1
   Compiling mithril v0.7.0 (file:///Users/yuanlinfeng/Work/Monero/mithril)
error: 128-bit type is unstable (see issue #35118)
  --> src/byte_string.rs:61:26
   |
61 | pub fn u128_to_string(u: u128) -> String {
   |                          ^^^^
   |
   = help: add #![feature(i128_type)] to the crate attributes to enable

error: 128-bit type is unstable (see issue #35118)
   --> src/cryptonight/hash.rs:134:14
    |
134 |     let r0 = u128::from(a.0);
    |              ^^^^^^^^^^
    |
    = help: add #![feature(i128_type)] to the crate attributes to enable

error: 128-bit type is unstable (see issue #35118)
   --> src/cryptonight/hash.rs:135:14
    |
135 |     let r1 = u128::from(b.0);
    |              ^^^^^^^^^^
    |
    = help: add #![feature(i128_type)] to the crate attributes to enable

error: 128-bit type is unstable (see issue #35118)
  --> src/u64x2.rs:65:29
   |
65 |     pub fn to_u128(self) -> u128 {
   |                             ^^^^
   |
   = help: add #![feature(i128_type)] to the crate attributes to enable

error: 128-bit type is unstable (see issue #35118)
  --> src/u64x2.rs:66:21
   |
66 |         let mut r = u128::from(self.1);
   |                     ^^^^^^^^^^
   |
   = help: add #![feature(i128_type)] to the crate attributes to enable

error: 128-bit type is unstable (see issue #35118)
  --> src/u64x2.rs:68:14
   |
68 |         r |= u128::from(self.0);
   |              ^^^^^^^^^^
   |
   = help: add #![feature(i128_type)] to the crate attributes to enable

error: aborting due to 6 previous errors

error: Could not compile `mithril`.

To learn more, run the command again with --verbose.
```