Activation meta-fields

# Definitions - **Activation**: The runtime object representing a method's scope. # The need - WebAssembly should allow reading/inserting meta-fields from/into activations. For example, `activation[[DefaultXMLNamespace]] = ""`. - WebAssembly should allow looking up for the nearest activation, in the scope chain, that defines a specific meta-field. # More context https://langdev.stackexchange.com/q/4713