Global environment in the specification

[38a938c2]

As the reference implementation defines global-environment to extract a global environment corresponding to the lexical environment, and this is an implicit assumption in the specification, add some explicit mentions of that into the specification.

Apparently global environment is expected to support the same protocol as the lexical environment (and more than that); not sure about the global-environment request itself. (Should the property of being global determined by global-environment returning the same object? Nil? By a special predicate? By being a subclass of a specified class?)

Probably also mention that the expected behaviour of «language processor» third-party libraries is to query a lexical environment, and fall back to explicitly querying the corresponding global environment in case of failure.