Add support for Normed construction from Rational (Fixes #157)

[kimikage]

This also fixes the overflow problem with Fixed construction from Rational. (Fixes #157)

Note the input range that Fixed allows (cf. issue #156).

[codecov-io]

Codecov Report

Merging #169 into master will decrease coverage by 0.31%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master     #169      +/-   ##
==========================================
- Coverage   88.64%   88.32%   -0.32%     
==========================================
  Files           5        5              
  Lines         370      377       +7     
==========================================
+ Hits          328      333       +5     
- Misses         42       44       +2

Impacted Files	Coverage Δ
src/normed.jl	90.05% <100%> (+0.17%)	⬆️
src/fixed.jl	87.34% <60%> (-2%)	⬇️

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[kimikage]

I chose the input range just because it is looser (i.e. it has a low impact of the change).
@timholy, if it is not too much trouble, please tell me what the input range should be.

[kimikage]

Of course, anyone's opinion is fine. I'm worried about setting the specifications against the thoughts of "silent" users. However, I also think that this is not a good place for consensus building.

[kimikage]

I cannot see the negative effect unless we release it. I'm concentrating my efforts on releasing v0.8.0.