Allow limiting the size of variable length types

For example, say you want a string that is max 10 bytes long. Should this be orthogonal to requesting that lengths be less that u64?

The issue names no files, tests, or entry points to inspect. Start by reading the repository's type and serialization APIs, then clarify whether variable-length values should support byte limits independently of u64 length limits; the work is done when the intended constraint behavior and tests are specified.

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