[michaelstaib]

This continues the work to execute a field shared by sibling deferred fragments once. There is no behavior change and no change to what is sent.

This branch merges #187 (interned defer usage sets) because the table is built on the interned sets.

Per-request defer usage set table:

• A request with incremental parts gets a table on its root operation context. The table maps each selection's interned defer usage set to the set that is active for the request's @defer(if:) values. A disabled usage folds into its nearest enabled parent, duplicates are removed, and a usage nested below another usage of the set is dropped.
• Each set is resolved once per request. Equal active sets are one instance within the request. Selection sets compiled while the request runs are picked up as well.
• Reading a resolved set does not lock and does not allocate.

Callers:

• The resolver task factory decides whether a field is deferred through the table. It still creates one deferred branch per active usage, in the same order as before.
• The stream page data requirement analysis uses the table and checks each connection child against the branch the connection runs in.

Stacked on #184.

Tests: DeferUsageSetTableTests cover the static, dropped, folded and pruned cases, per-request interning, and sets compiled after the table was created. Two allocation tests check that a list under a disabled or alternating @defer does not allocate per object. Execution.Tests pass on net10.0, the defer and stream tests pass on net8.0, and the cursor pagination and stream page connection suites pass unchanged.

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