Allow measurements to be implicitly converted

[alanhussey]

Adds methods to Measurement to allow them to be converted without explicitly referencing a UnitSystem.

m`48 inches`.as(foot)
m`48 inches`.in.feet

I think I like the "fluent" syntax this provides, but I'm not sure how I feel about the implementation. Defining a new class SystemMeasurement for each instance of UnitSystem is weird.

Another option is to make every Measurement-producing method on UnitSystem give each Measurement a reference to the system. .in and .as would then have to check whether that system was provided (guarding against user-created measurements with no access to system).

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This also introduces the possibility of embedding a mini-language in m. m`48 inches in feet` reads very nicely. I'm wary of trying to going even further down this path, however.