World age errors with --compiled-modules=no

[c42f]

Various of the Unitful tests are broken when using julia --compiled-modules=no, due to the definition order of functions and methods relative to generated functions. For example

Construction: Error During Test at /home/tcfoster/.julia/dev/Unitful/test/runtests.jl:57
  Test threw exception
  Expression: 3 * (ContextUnits(m) / ContextUnits(m)) === 3
  MethodError: no method matching ^(::Unitful.Unit{:Meter,𝐋}, ::Rational{Int64})
  The applicable method may be too new: running in world age 25620, while current world is 26541.
  Closest candidates are:
    ^(::Unitful.Unit{U,D}, ::Number) where {U, D} at /home/tcfoster/.julia/dev/Unitful/src/units.jl:105 (method too new to be called from this world context.)
    ^(!Matched::Missing, ::Number) at missing.jl:97
    ^(!Matched::Irrational{:ℯ}, ::Rational) at mathconstants.jl:91
    ...
  Stacktrace:
   [1] (::getfield(Unitful, Symbol("##4#5")))(::Unitful.Dimension{:Length}) at ./array.jl:0
   [2] iterate(::Base.Generator{Tuple{Unitful.Dimension{:Length}},getfield(Unitful, Symbol("##4#5"))}) at ./generator.jl:47
   [3] append_any(::Any, ::Vararg{Any,N} where N) at ./essentials.jl:674
   [4] #s5#3(::Any, ::Any, ::Any) at /home/tcfoster/.julia/dev/Unitful/src/user.jl:306
   [5] (::Core.GeneratedFunctionStub)(::Any, ::Vararg{Any,N} where N) at ./boot.jl:522
   [6] upreferred at /home/tcfoster/.julia/dev/Unitful/src/user.jl:329 [inlined]
   [7] Unitful.ContextUnits(::Unitful.FreeUnits{(m,),𝐋,nothing}) at /home/tcfoster/.julia/dev/Unitful/src/types.jl:107
   [8] top-level scope at /home/tcfoster/.julia/dev/Unitful/test/runtests.jl:57
   [9] top-level scope at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.1/Test/src/Test.jl:1083
   [10] top-level scope at /home/tcfoster/.julia/dev/Unitful/test/runtests.jl:38

This seems to be the same root cause as #231 (ie, methods not defined at the time of generated function definition) but the fix is arguably a bit different as it requires changing the order of how functions are defined within Unitful itself.

I tried doing this, but I got confused by the interdependence between upreferred(:: and unary *(::FreeUnits). The backtraces seemed misleading.

[ajkeller34]

This is probably the next issue I'll tackle. I can get most of the tests to pass already but there are some subtle bugs that can give incorrect results without throwing MethodErrors.

[ajkeller34]

Fixed by #243