Added new IGNORE constant

[jwoertink]

Fixes #1017

This PR adds a single Avram::Nothing.new instance assigned to a new constant called IGNORE. This does 2 things

1. Uses a single allocated instance everywhere to represent Avram::Nothing instead of a new instance for every column. While this doesn't give a ton of memory savings, it's still better
2. For those nasty overload errors, it shrinks them up by a few characters.

Before:

After:

[robacarp]

🚀