WASM: Support moving subtrees

Context

For discussion in the SCORU WASM crew. This MR solves the "swap issue" where things move around in the tree.

Here's an example of how the new encoding works for a scenario where two vectors swap places in the tree.

Start with a tree (T1):

T1:
  - foo
    - 0 : A
    - 1 : B
    - 2 : C
  - bar
    - 0 : X
    - 1 : Y

Run a decode step to decode the tree into a record {foo : vector; bar : vector}

Note that this does not read any values from the tree.

v1 = { tree: T1/foo; produce_value = read_from T1/foo; values = {}; }
v1 = { tree: T1/bar; produce_value = read_from T1/bar; values = {}; }
value1 = {foo = v1; bar = v2}

Now flip v1 and v2:

value2 = { foo = V2; bar = V1}

Encode the new value given the old tree T1:

• Remove foo from T1
• Add the tree from vector v2 to foo
• Add the loaded bindings from v2 to foo
• Remove bar from T1
• Add the tree of vector v1 to bar
• Add the loaded bindings from v1 to bar

The new tree T2 now has the shape:

T2:
  - foo
    - 0
    - 1
  - bar
    - 0
    - 1
    - 2

Manually testing the MR

CI.

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.
• Select as Assignee the next person who should take action on that MR