enum messages: Make the returned values all 'static

[ijackson]

This is (at least in theory) a breaking change, because someone might
have implemented this trait by hand, and their implementation won't
be declared as returning 'static lifetimes.

Closes #159

[ijackson]

The Travis failure CI failures seem unrelated:

error[E0658]: use of unstable library feature 'alloc': this library is unlikely to be stabilized in its current form or name (see issue #27783)

   --> /home/travis/.cargo/registry/src/github.com-1ecc6299db9ec823/enum_variant_type-0.2.1/src/lib.rs:144:1

    |

144 | extern crate alloc;

    | ^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0658`.

error: Could not compile `enum_variant_type`.

[Peternator7]

Fixed the build with #162, could you merge master and re-push?

This seems generally useful to me so I'm okay taking the small chance of breaking existing code.

[ijackson]

The AppVeyor failure is this:

verbose: creating temp file: C:\Users\appveyor\.rustup\tmp\5vzkbz8b4rvu6tfk_file
verbose: creating temp file: C:\Users\appveyor\.rustup\tmp\5wypi_xgi7diffwp_file
verbose: deleted temp directory: C:\Users\appveyor\.rustup\tmp\b658ydoei39unwji_dir
info: installing component 'rust-std'
verbose: creating temp directory: C:\Users\appveyor\.rustup\tmp\h32i514dflmtq_j2_dir
memory allocation of 16777216 bytes failed
Command exited with code -1073740791

The error code is 0xffffffffc0000409. No idea what that means. I don't think this is related to my changes.

[Peternator7]

Appveyor has been flakey lately. Re-running fixed the gate