Work around another ambiguity and centralize floattype, nbitsfrac, an…

[timholy]

…d rawtype.

Only two ambiguities left (signbits and typechar), due to JuliaLang/julia#20246.

[codecov-io]

Codecov Report

Merging #75 into master will increase coverage by 1.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #75      +/-   ##
==========================================
+ Coverage   83.42%   84.44%   +1.01%     
==========================================
  Files           4        4              
  Lines         181      180       -1     
==========================================
+ Hits          151      152       +1     
+ Misses         30       28       -2

Impacted Files	Coverage Δ
src/fixed.jl	74.07% <ø> (+4.07%)	✅
src/normed.jl	90.36% <ø> (-0.45%)	❌
src/FixedPointNumbers.jl	90.47% <100%> (+1%)	✅

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 872675d...486edf4. Read the comment docs.