Consider supporting negative scale for decimal type

(Relates to ODATA-771)

ODATA-789 attempted to address a number of questions around the use of Edm.Decimal. Part of that was clarifying/correcting current semantics of Edm.Decimal, and was accepted at part of ODATA-789. Part of the issue discussed the addition of an exponential notation, which was already the topic of ODATA-771 and will be tracked there.

This issue tracks the remaining issue from ODATA-789, the proposal to expand Edm.Decimal to support negative scale.

Decimal data types in programming languages and databases
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Common terminology is

Value = Mantissa * 10**(-Scale)
Precision = length of Mantissa

Positive Scale means fractional digits, negative scale avoids representing "trailing zeroes" for large integers, e.g. one million has mantissa = 1, scale = -6

Type Precision = length of mantissa Scale = -Exponent
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C# decimal 96 bit ~ 28-29 decimal digits Instance, 0...28
Objective-C NSDecimalNumber 38 decimal digits Instance, -128...127
Java BigDecimal unlimited no of decimal digits Instance, -2,147,483,648...2,147,483,647 (32 bit)
DECFLOAT34 34 decimal digits Instance, -384...383
DECFLOAT16 16 decimal digits Instance, -6144...6143
DB2 DECIMAL 1...31 decimal digits Column, 0...Precision
Sybase ASE DECIMAL 1...38 decimal digits Column, 0...Precision
Sybase IQ DECIMAL 1...126 decimal digits Column, 0...Precision
Postgres DECIMAL 1...1000 decimal digits Column, 0...Precision
Oracle NUMBER 1...38 decimal digits Column, -84...127

Problem when mapping NUMBER to Edm.Decimal

• Scale cannot be negative or larger than Precision