Add cord data structure (#2273)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2273

Introduce cord data structure to store bytes/bytearrays during serialization.

This allows us to manipulate bytes/bytearrays without copying data.

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Reviewed By: dbort

Differential Revision: D54514244

fbshipit-source-id: 65397dcdea93054d54feea1b9f3ebfb0940c8513