Support constants that could be used along with a grouping

[brharrington]

For some use-cases, it might be helpful to support a form of constant that could be used to set different values for each result of a group by. Example:

name,foo,:eq,(,region,),:by,
(,region,east,4,region,west,5,),:const,(,region,),:by,:gt

The exact syntax and operators would need to be worked out. It would likely need to support some form of default value that could be used for entries that weren't explicitly in the mapping.

/cc @ruchirj