Optimize Fixed --> Float conversions

[kimikage]

Closes #171.

[kimikage]

Of course, there are minor breaking changes regarding the improvement of numerical errors.

julia> Float32(-0x1p-25)
-2.9802322f-8

julia> Float32(-eps(Q6f25)) # error (on FixedPointNumbers v0.7.1)
0.0f0

julia> Float32(eps(Q6f25))
2.9802322f-8

[codecov-io]

Codecov Report

Merging #172 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #172      +/-   ##
==========================================
+ Coverage   88.32%   88.39%   +0.06%     
==========================================
  Files           5        5              
  Lines         377      379       +2     
==========================================
+ Hits          333      335       +2     
  Misses         44       44

Impacted Files	Coverage Δ
src/fixed.jl	87.65% <100%> (+0.31%)	⬆️

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