[DISCUSS] Reduce use of freemarker templates for functions

[Mehant Baid]

Hey All,

We use freemarker templates extensively for generating source code for 
value vectors and functions. However over time the template logic for 
functions has become complicated with the need to modify the function 
templates based on input data type, nullability, function type etc. This 
makes the template code extremely hard to read and debug. Proposal going 
forward is to de-templatize the functions and check-in the source for 
various functions (like how we have been doing in recent times for hash 
functions etc). Although its not ideal as it involves some code 
duplication but the advantage is that the source is then 'IDE friendly' 
and much more readable. As part of the work for DRILL-2870 I am planning 
to de-templatize the aggregate functions.

Thanks
Mehant