Minor improvements required to build and/or extend and/or modify Compilers

[mindcrash]

While working around some issues within the Postgres compiler (like not supporting some operations required to query arrays and JSON) I discovered some issues which can be worked around right now to a certain degree but are really annoying.

1. Some methods which could be required for modifications are marked internal or private, making them inaccessible from the outside (while you might need them). At least make sure these methods are marked protected.
2. The parameter placeholder is coded hard in at several places, making it impossible to make the compiler work when overriding it when copying all other code using it and changing it manually.

If you are unable to clean this up yourself I am happy to prep and submit a PR once I have some free time to do so.

[generik0]

Why not clean it and make a PR?
That is what I would do :-)

[ahmad-moussawi]

@mindcrash could you please provide a list of the methods that you need to change, maybe we can discuss this more.