WIP: adds a test-case for lowering/lifting of 64bit string functions

[christoph-dfinity]

This PR currently just adds a failing test showing that string parameters are currently lowered to i32 i32 even when targeting a 64bit memory.

---- "tests/cli/component-model/memory64/string.wast" ----
failed test "tests/cli/component-model/memory64/string.wast"
...
       Caused by:
           0: failed testing wasm binary produced by `wast`
           1: wasm isn't valid
           2: lowered parameter types `[I32, I32]` do not match parameter types `[I64, I64]` of core function 0 (at offset 0x8f)

From my reading of the Canonical ABI document, I'm thinking this test should pass.

Strings are loaded from two pointer-sized values ...

If someone could confirm, I'd be happy to work on an implementation.

/cc @michael-weigelt