Lens module memory management is broken

[AndrewSisley]

At the moment, the allocation of memory for input params is owned by the host, yet the freeing is handled by the module.

The return param allocation is managed by the module, but the freeing is not done explicitly (this might be a rare race condition, or a leak).

I think we need to add a new module free(ptr, size) function. The host should call into the wasm module to free input and return values, and allocate input memory. Allocating return value mem can remain within the module.

New Lens tests need too be written, as there is at least one test gap here, that was only caught by defra: TestSchemaMigrationQueryMigrationCopiesExistingFieldWhenSrcFieldNotRequested - relied on a previous memory leak encouraged by the current model.