4.5.1 - Refactor encoder property to weak reference in KeyedContainerImpl and UnkeyedContainerImpl to prevent retain cycles

What's Changed

Refactor encoder property to weak reference in KeyedContainerImpl and UnkeyedContainerImpl to prevent retain cycles by @makoni in #239

I’ve noticed memory usage growth in my project that is described in vapor/leaf#238

After adding some tests and running leaks, I found that currently there are retain cycles. That PR fixes them.

  4 (352 bytes) ROOT CYCLE: <EncoderImpl 0x1572270d0> [80]
     3 (272 bytes) storage --> ROOT CYCLE: <Leaf.LeafEncoder.(KeyedContainerImpl in _30C79F4310B915184892E56903FC555A)<CodingKeys in Foo #1 in closure #1 (Vapor.Application) async throws -> () in LeafTests.LeafMemoryGrowthTests.testRepeatedRenderMemoryGrowth() async throws -> ()> 0x157227120> [48]
        __strong encoder --> CYCLE BACK TO <EncoderImpl 0x1572270d0> [80]
        2 (224 bytes) __strong data._variant --> <Swift._DictionaryStorage<Swift.String, Leaf.(LeafEncodingResolvable in _30C79F4310B915184892E56903FC555A)> 0x157227170> [192]  item count: 1
           1 (32 bytes) <Closure context 0x157227150> [32]

  2 (128 bytes) ROOT CYCLE: <EncoderImpl 0x156710ad0> [80]
     1 (48 bytes) storage --> ROOT …

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This patch was released by @gwynne

Full Changelog: 4.5.0...4.5.1