Improve StaticArrays usage

[wsshin]

This PR makes a few improvements on the usage of StaticArrays:

• Halve the size of signmatrix to work around StaticArrays Issue #384.
• Simplify some expressions by taking advantage of StaticArrays PR #274 on broadcast.

[codecov-io]

Codecov Report

Merging #27 into master will not change coverage.
The diff coverage is 60%.

@@           Coverage Diff           @@
##           master      #27   +/-   ##
=======================================
  Coverage   93.72%   93.72%           
=======================================
  Files           7        7           
  Lines         223      223           
=======================================
  Hits          209      209           
  Misses         14       14

Impacted Files	Coverage Δ
src/ellipsoid.jl	100% <ø> (ø)	⬆️
src/box.jl	93.75% <60%> (ø)	⬆️

---

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 211ec40...1f6b786. Read the comment docs.