Improve Publish efficiency by avoid allocations.

[schungx]

Changes:

• Abstract out Payload with a minimum fixed buffer of 8 bytes. Anything larger goes into a Vec<u8>.
• Standard conversions of standard types to Payload.
• Use Cow<'static, str> for Publish topic to enable using constant strings as topics.

Sorry, since you don't have a rustfmt.toml, it messes up your formatting a bit.

[fluffysquirrels]

As discussed in #11, I'm going to close this without benchmarks to show they speed things up.