Don't panic for try_lift errors

[bendk]

This avoids crashing for consumers that set panic=abort (Firefox).

• Updated the Err type for rust_call. The closure can now return an internal error. The only way to signal an internal error before was to panic.
• Reworked the LowerReturn trait and the future code to work with the new system.
• Added a type for argument lift errors, this makes the rustfuture signatures nicer.

[pierre-wehbe]

@mhammond Would this allow flat_errors to be lifted by any chance?

Can't lift flat errors
rustPanic("Can\'t lift flat errors")

[mhammond]

@mhammond Would this allow flat_errors to be lifted by any chance?

I doubt it - that's largely impossible IIUC - consider that we allow a flat enum to have any type - eg

enum Foo {
  Variant(SomeOtherType),
}

Where SomeOtherType can be literally anything that works in Rust - it doesn't need to be a Uniffi type. This works because it's flat - we just lower the variant discriminator without SomeOtherType at all. This makes it almost impossible to lift - lifting implies somehow we are able to create Variant(SomeOtherType) without a value for SomeOtherType.