Apply WHERE clause to INSERT SELECT .. ON CONFLICT

[jkachmar]

INSERT SELECT .. ON CONFLICT .. UPDATE is currently implemented as insertSelectWithConflict, which provides basic upsert functionality in a fairly nice way.

Unfortunately it's fairly limited compared to Esqueleto's update in that it's not possible to constrain what gets updated with a where_ clause.

The current insertSelectWithConflict is nice in that the third argument has the signature:

(SqlExpr (Entity val) -> SqlExpr (Entity val) -> [SqlExpr (Update val)])

...which provides a concrete list of updates, which can be used to choose between DO NOTHING or DO UPDATE when the SQL is being generated.

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I'm mostly opening this issue up so that interested parties can discuss and/or propose a good API for this. From what I've poked around with so far, everything has small compromises in terms of ergonomics or type safety.

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Related issues/PRs: #131 #155