WASM: use a lazy_vector for block parser as continuation stack

Context

The WASM blocks parser actually use a list of continuations when parsing blocks. This MR simply replaces it by a lazy_vector that act as a stack: we retain an offset on the vector that points to the top of the stack, push and pop are responsible to update it when necessary.

This vector should be easy to encode in Irmin: since there's never two block_kont at the same time, the same key can be reused everytime for the continuation stack.

Manually testing the MR

Checklist

• 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).
• Select suitable reviewers using the Reviewers field below.
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