Trap toplevel suspensions

[dhil]

Patch #37 provides an implementation of the Wast directive assert_suspension, enabling us to run test cases for unhandled tags. However, our current implementation traps differently depending on whether the suspension occurs in a continuation context or the main stack. In case of the latter it triggers an unrecoverable panic, which the assert_suspension cannot (and should not!) handle. Consequently, we are unable to run test cases with toplevel suspensions at the moment.