Add support for executable rule inputs

[kislayverma]

Instead of rule inputs being fixed values, we should allow them to be arbitrarily executable pieces of code. This will allow definition of custom logic instead of static values.

A few things to consider here.

1. Performance may be impacted if the logic is intensive.
2. What data be passed to these inputs and how.
3. Does this mean a new rule input type (e.g. 'EVAL') and how does that play with the data type construct.

This is also a significant deviation from the current simple approach of Rulette towards rule inputs, viz that of treating them as stored "values". With this, we shift more towards Drools style rule "execution", although we may be able to support both modes.

[kapilratnani]

The executable rules can have some side-effect or are just pure function returning a value?

[kislayverma]

No side Effects. They will comply with the Rule input contract and use "some" data. What should that data be though, is the question