package modernization including better type stability

[ExpandingMan]

Seemed like this package needed a bit of an overhaul as it appears to be basically the default arbitrary precision decimal arithmetic package. It still needs a ton of work, but I think this is a good start. The most significant improvement is in type stability as the fields of Decimal are now concrete types.

At first I was going to make Decimal parametric with a type parameter for the exponent, but ultimately I decided that the benefit if this was pretty dubious, so I set the exponent to simply be an Int. Because of this, these changes should be pretty much non-breaking.

[coveralls]

Pull Request Test Coverage Report for Build 113

• 12 of 15 (80.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-2.6%) to 92.638%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/decimal.jl	6	9	66.67%
<!--	Total:	12	15

Totals
Change from base Build 111:	-2.6%
Covered Lines:	302
Relevant Lines:	326

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💛 - Coveralls

[codecov-io]

Codecov Report

Merging #42 into master will decrease coverage by 2.55%. The diff coverage is 80%.

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
- Coverage   95.28%   92.72%   -2.56%     
==========================================
  Files           6        6              
  Lines         106      110       +4     
==========================================
+ Hits          101      102       +1     
- Misses          5        8       +3

Impacted Files	Coverage Δ
src/round.jl	91.66% <ø> (ø)	:arrow_up:
src/arithmetic.jl	80% <100%> (ø)	:arrow_up:
src/equals.jl	100% <100%> (ø)	:arrow_up:
src/Decimals.jl	100% <100%> (ø)	:arrow_up:
src/decimal.jl	90.47% <66.66%> (-6.9%)	:arrow_down:

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