Decimal Writer Support

[wilwade]

Problem

Need to support writing decimal types

Also closes: #87

Solution

Add basic encoding support for decimals

Change summary:

• Added better decimal field errors
• Default scale to 0 per spec
• Cleanup types on RLE so it only asks for what is needed
• Support decimal encoding
• Add test for write / read of decimal

Steps to Verify:

1. Generate a schema with decimal field
2. Use it!

[JasonYeMSFT]

The changes look good regarding #87. Please feel free to close the issue once this PR is merged.