use a non-initializing allocator for (de)serialization buffers

[froydnj]

Motivation

Fewer dead writes to memory during deserialization, which should theoretically be faster.

(The same principle can be applied to the serialization buffers, but that is a more invasive change. At the moment this is more of a gut check.)

Test plan

See included automated tests.

[froydnj]

We have a policy of testing changes to Sorbet against Stripe's codebase before merging them. I've kicked off a test run for the current PR. When the build finishes, I'll share with you whether or how it failed. Thanks!

Stripe employees can see the build results here:

→ https://go/builds/bui_MAKEsKS3KmIy6P → https://go/builds/bui_MAKER43nTmgSur