This PR adds optional support for preserving the shadow stack when using the wasmfx implementation of the fiber interface.

This is achieved as follows:

• for each continuation, we malloc a dedicated region of linear memory, to be used as its shadow stack
• for each continuation, we save 1) the beginning of this region of memory and 2) the current shadow stack pointer (within that region)
• whenever we resume a continuation, we set the global shadow stack pointer aside and set it to the previous shadow stack pointer saved for the continuation to be resumed
• once a continuation finishes (via return or suspend), we restore the global stack pointer. Further, unless the continuation has returned, we also update the shadow stack pointer of the suspended continuation using the value of the global shadow stack pointer at the point of the suspend.