Make some small style/effciency improvements

[jlapeyre]

• Use mutating eigvals! to avoid an allocation.
• Add comment on reason for promotion.
• Use fill rather than ones. This is cleaner and more efficient.

Although these changes include improvements in efficiency, I found no measurable difference
in run time. It's more a question of best practices. Eg, you should avoid an allocation
if it doesn't make the code less readable.

I meant to include these in the previous PR, but they escaped somehow.

[codecov]

Codecov Report

Merging #12 into master will not change coverage by %.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #12   +/-   ##
=======================================
  Coverage   96.96%   96.96%           
=======================================
  Files           2        2           
  Lines          33       33           
=======================================
  Hits           32       32           
  Misses          1        1           

Impacted Files	Coverage Δ
src/char-values.jl	96.87% <100.00%> (ø)

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[matthewware]

LGTM. Thanks again John!