[WIP] Serialization traits for data dependencies

[grynspan]

Experimental .serialized(for:) trait. Example use cases:

import Foundation

@Test(.serialized(for: ProcessInfo.self))
func `HAS_FREEZER environment variable`() {
  _ = setenv("HAS_FREEZER", "1", 1)
  #expect(FoodTruck.hasFreezer)
  _ = setenv("HAS_FREEZER", "0", 1)
  #expect(!FoodTruck.hasFreezer)
}

@Test(.serialized(for: *))
func `Uses lots of different state, seriously, just scads of it`() { ... }

Name subject to extreme bikeshedding!

Resolves rdar://135288463.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

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