Implement Wasm proposal: `extended-const`

[Robbepop]

This is about adding support for the extended-const Wasm proposal.

The extended-const Wasm proposal has been recently moved to Phase 4 and thus stabilization can be expected to happen soon. Furthermore it is a fairly light weight proposal, therefore implementing it won't be a lot of work.

Having extended-const support in wasmi will be an enabler for use cases that rely on dynamic linking of Wasm modules.

[Robbepop]

Implemented in #707. Closed.