Make DSLs immutable

[badgerwithagun]

Currently, most elements of the SQL DSL are mutable. This is one of those things that occurred organically over time to make the DSL read more cleanly, but which has increasingly looked out of place. It also prevents a number of performance optimisations.

All SQL dialect elements need to be made immutable, allowing us to resolve some unpleasant API side-effects. Once this is done, we can significantly improve caching and performance.

This will have widespread binary compatibility issues, and even text API compatibility in some edge cases.

[badgerwithagun]

First stage is available for merge at #43.

Working on the next stage on https://github.com/badgerwithagun/morf/tree/immutable-statements

[badgerwithagun]

Second stage now available for merge in #47.

[badgerwithagun]

After this, we're going to have to start breaking compatibility.

[badgerwithagun]

#47 merged.

[badgerwithagun]

@pstefaniak7 is currently working on the last steps in making Alfa's internal code compatible with "immutable mode". Once that's done we'll flip the default behaviour around and start removing mutable support.

[therealluigi]

@pstefaniak7 what is the progress on this one? Finished? Abandoned?

[pstefaniak7]

@therealluigi
No progress on this since last update in 2018.