Update to run on rust nightly 1.15.0

[acmiyaguchi]

llamadb is broken in the current rust nightly 1.15.0 release.

The first change is using the Cow:: smart pointer constructor instead of into_cow, since it has been depreciated. The other change is to the time::Duration module, where span was depreciated. The idiomatic way to time a function is to use Instant::now() with the corresponding elapsed() function.

[stalkerg]

It can be working on the latest stable Rust release?

[acmiyaguchi]

@stalkerg I took a look at it and I'm not sure if I want to mess around with it. Associated types are behind a feature gate and are being used in the parser for generic programming. I'd rather spend my time doing some other things (namely adding tests).

For more reference, here's the tracking issue for associated types if you're curious on how they work and want to tackle it yourself.