Add special case comparisons of Decimal with NaN/Inf

[Keno]

Inf is a common starting value for min comparisons (and similarly
for -Inf and max). As an example Plots.jl uses it to figure out
the extrema of values to be plotted. Since Inf is not representable
in Decimal, these comparisons were throwing an error. Fix this by
adding special case comparisons with AbstractFloat that behave properly
for the Inf case.

[coveralls]

Pull Request Test Coverage Report for Build 94

• 9 of 13 (69.23%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-2.5%) to 92.079%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/equals.jl	9	13	69.23%

Totals
Change from base Build 91:	-2.5%
Covered Lines:	93
Relevant Lines:	101

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 94

• 9 of 13 (69.23%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-2.5%) to 92.079%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/equals.jl	9	13	69.23%

Totals
Change from base Build 91:	-2.5%
Covered Lines:	93
Relevant Lines:	101

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 96

• 13 of 13 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.5%) to 96.04%

---

Totals
Change from base Build 91:	1.5%
Covered Lines:	97
Relevant Lines:	101

---

💛 - Coveralls

[ararslan]

Very nice, thanks!