Let `WasmTy` differentiate between passing ownership into Wasm vs a borrow

Only matters for reference types. But if we know the call is borrowing a reference type that will outlive the call, we don't need to add it into the activations table because the borrow checker already ensures it will live long enough.

Currently only `Func::{wrap,get}` will pass by value (and therefore need the activations table) while `Func::call` will pass borrows (and therefore we know the reference will outlive the call, and it doesn't need to go into the table).

See also https://github.com/bytecodealliance/wasmtime-dotnet/issues/27#issuecomment-662002571

Start by tracing WasmTy and the Func::{wrap,get,call} paths described in the issue, then inspect how reference arguments enter the activations table. Confirm the existing ownership and borrow behavior before deciding how the type distinguishes them. Done means borrowed reference types passed through Func::call no longer require activation-table entries, while passed-by-value references still do.

Written by the indexing model from the issue text.