Correct macros to generate valid 2015/2018 edition code.

[iliekturtles]

Doc test on the $quantities! macro would fail with the error no ``mks`` external crate when the system! macro was invoked in a crate
using the 2018 edition. All invocations of the $quantities! macro are
corrected and a new test crate, edition_check, is added to validate
uom can be used in 2018 edition code. Resolves #119.

@svartalf if you could review that would be great.

[iliekturtles]

I ended up asking for a review too soon. I forgot that older versions don't support crate in paths. Trying to find a solution that supports old rustc versions.

[coveralls]

Coverage remained the same at 97.077% when pulling c3898b8 on dev-2018 into 54cc327 on master.

[iliekturtles]

@svartalf Ready for a review in my dev-2018 branch if you have some time. I'll try to merge in the next few days and push out another version to crates.io after that.

[svartalf]

@iliekturtles what minimal rust version will be required by the "next" uom version?

If you're going to stick with 1.24 (same as now), I guess that your changes is the best option, since both crate and $crate are available only in >=1.30

I've tested this branch with my #119 code also, works flawlessly :)

[iliekturtles]

I plan to keep supporting 1.22.0 for compiling uom and 1.24.1 for full support for the foreseeable future. I didn't consider this requirement a big enough justification for a bump in the required version since there is an easy (although ugly) work-around.

Eventually the requirement will get bumped up. I'm doing some experimentation with proc-macros and how quantities track their units in the dev-macros branch (very rough still). I'm contemplating calling the current uom 1.0 and if that branch works out releasing it as 2.0.