Improve the performance of * and / operators

[Mordil]

Motivation:

The current implementation of the multiplicative arithmetic operators is naive, as it converts the minorUnits
into a Decimal representation, applies the artihmetic, then does a conversion back to minorUnits.

This can be improved with conversions to both Double, and by doing the arithmetic directly.

Modifications:

• Change: * and / operators to do conversions to Double instead of Decimal, and inline the arithmetic operations needed

Result:

Performing multiplicative arithmetic against currencies should be much faster now. (XCTest measure reports 50% improvement)

[codeclimate]

Code Climate has analyzed commit cb644de and detected 0 issues on this pull request.

View more on Code Climate.

[codecov]

Codecov Report

Merging #30 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
+ Coverage   99.36%   99.36%   +<.01%     
==========================================
  Files           6        6              
  Lines         469      472       +3     
==========================================
+ Hits          466      469       +3     
  Misses          3        3

Impacted Files	Coverage Δ
Sources/Currency/AnyCurrency.swift	96.87% <100%> (+0.15%)	⬆️

---

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 cfc56f8...cb644de. Read the comment docs.

[Mordil]

Methodology:

• Alternated operators being tested in between each test
• Code snippet:

  for i in 1...1_000_000 {
      _ = USD(minorUnits: i) <operator under test> USD(minorUnits: i)
  }

• Executed each test 3 times

Before:

Operator	Run 1	Run 2	Run 3	Average
*	2.064	1.968	1.970	2.002
/	2.399	2.404	2.419	2.407

After:

Operator	Run 1	Run 2	Run 3	Average
*	1.073	1.071	1.062	1.069
/	1.076	1.145	1.078	1.100