Context

Split off work from recent MRs I've been working on, to make future reviews more digestible.

Adds some features to lazy map and chunked byte vector in addition to a test suite for both.

Manually testing the MR

dune test src/lib_webassembly

Document the interface of any function added or modified (see the coding guidelines)
Document any change to the user interface, including configuration parameters (see node configuration)
Provide automatic testing (see the testing guide).
For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
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