Implement SQL-consistent null and boolean handling

[rw-access]

Part of #15

We need to implement three-value logic for null, true and false consistent with Elasticsearch EQL and Elasticsearch SQL.

Some necessary changes

• Any value can be compared to null
• The only way to explicitly check for null is == null or != null (plus commutative versions)
  • Likely need an intermediate ast node to delineate these nulls from folded nulls
• All other comparisons to null return null (< null, <= null, etc)
• Dynamic nulls can not be compared. process_name == parent_process_name will be null even if both values are null. This is a change from the current implementation
• Implement boolean logic similarly, while maintaining the short-circuiting properties: ... and false => false, and ... or true => true
• Disable implicit boolean casting. We can keep a toggle to manually turn it back on
• Functions with any null required inputs return null
• Might be worthwhile to track functions that can return null, even when all parameters are the correct-type and non-null

And of course, update existing tests to be consistent