`promote_type(Measurement{Float64},BigFloat)` fails

[longemen3000]

Found while testing #178
MVE:

using Measurements
promote_type(Measurement{Float64},BigFloat)

Fails with:

ERROR: StackOverflowError:
Stacktrace:
      [1] promote_result(::Type, ::Type, ::Type{Measurement{BigFloat}}, ::Type{BigFloat})
        @ Base .\promotion.jl:338
      [2] promote_type
        @ .\promotion.jl:318 [inlined]--- the above 2 lines are repeated 79981 more times ---