Produce exponential (string) representation of numbers

[jessealama]

As it stands, the toString method in the Decimal128class produces a complete "read out" of each digit of a number. Please add a new method that allows one to get a string representation of a Decimal128 value in exponential format.

Examples that should work:

• 1.2 → 1.2e-1
• 100 → 1e2
• 42 → 4.2e1
• 1 → 1e0
• -123.456 → -123456e-3