…ruct (swiftwasm#800)

…ics-import

BridgeJS: Support generic functions on imported JS APIs

An imported optional whose payload is stack-only ([T]?, [String: V]?,
@js struct?) used a hybrid convention: the isSome flag crossed as a wasm
i32 parameter while the payload was conditionally pushed onto the shared
stacks. Optional returns and optional array elements of the same types
already travel entirely on the stacks: payload first, then a 0/1 flag on
the i32 stack.

This lowers those parameters the same way. The Swift thunk pushes the
payload (if some) followed by the flag, the wasm signature carries no
argument for the parameter, and the JS handler pops the flag before
conditionally lifting the payload, through the same fragment already
used for optional returns and elements.

The hybrid shape was the last parameter category that both passed a wasm
argument and pushed stack data, which is what enabled the argument
transposition fixed in swiftwasm#794. Every stack-touching parameter is now
flagless and reverse-ordered, matching returns and elements. All other
optional parameter ABIs (scalars, strings, JSObject, closures, enums,
heap objects) are unchanged.

…stack-flag

BridgeJS: Lower imported optional stack parameters fully on the stack