Improve mid and bisect

[dpsanders]

Fixes #243

[codecov-io]

Codecov Report

Merging #261 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #261      +/-   ##
==========================================
+ Coverage   90.26%   90.31%   +0.04%     
==========================================
  Files          26       26              
  Lines        1079     1084       +5     
==========================================
+ Hits          974      979       +5     
  Misses        105      105

Impacted Files	Coverage Δ
src/intervals/arithmetic.jl	100% <100%> (ø)	⬆️
src/root_finding/bisect.jl	100% <100%> (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 1c0b48f...82890f9. Read the comment docs.

[coveralls]

Coverage increased (+0.04%) to 90.314% when pulling 82890f9 on improve_bisect into 1c0b48f on master.

[dpsanders]

OK to merge @lbenet ?

[lbenet]

Merging...