Define new return types for the view ABI

[jsdt]

Description of Changes

This adds some changes for how we return data from view functions. Originally, we interpreted the output of a view function as a bsatn encoding of an array of rows. Since we also want to be able to return queries from view functions, we need to be able to return different types too. At this point, this is effectively not a functional change, since we don't use the new format, and we don't actually try to parse the new format.

This introduces a new format for view returns, which is a ViewResultHeader, potentially followed by additional data. For example, if a view were returning rows directly, it would write a ViewResultHeader::RowData, followed by an array of rows. Note that we could have given that object a byte array with the rows instead of using a header an a separate object, but that would force us create an extra copy when encoding and when decoding.

To make this backward compatible with existing views, we have a different way to return the new format. For v8 views, if they return a byte array, we assume it is the old format. If they return an object, we expect the data field of that object to be the actual return data.

For wasm views, we interpret a return code of 2 to mean that it uses the new format.

There will be follow up PRs to use the new return type, and to actually handle the new format (and queries) on the host side.

API and ABI breaking changes

This is not a breaking change, but it does make the ABI more complicated (specifically to avoid breaking it).

Expected complexity level and risk

1.5. This should not affect the existing return style.

Testing