weak, structs: add comprehensive example functions

[mrbishalbaniya]

This commit adds example functions to the weak and structs packages to improve documentation and demonstrate usage patterns.

weak package examples:

• Example: Basic weak pointer usage
• ExampleMake: Creating weak pointers
• ExampleMake_nil: Creating weak pointers from nil
• ExamplePointer_Value: Accessing weak pointer values
• ExamplePointer_Value_garbageCollected: Behavior after GC
• Example_cache: Implementing a cache with weak pointers
• Example_canonicalization: String interning pattern
• Example_lifetimeTying: Tying object lifetimes together
• Example_equality: Weak pointer equality semantics

structs package examples:

• ExampleHostLayout: Basic HostLayout usage for C compatibility
• Example_cInterop: C struct interoperability
• Example_nestedStructs: HostLayout scope behavior
• Example_aliasing: Using HostLayout aliases

These examples demonstrate practical use cases including caches, canonicalization maps, lifetime management, and C interoperability.

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