Bring back SymbolLookup

When the foreign API was moved to java.base we initially left out SymbolLookup and moved some of its functionalities to CLinker and ClassLoader.

This moves makes it harder for clients to get a full picture of how to lookup symbols in loaded libraries. Moreover, following recent JDK changes (see JDK-8281335 and JDK-8282608), we can now offer a lookup which provides deterministic library loading/unloading.

Given we have three different ways to provide symbol lookups, and given that users might want to define their own lookup, by combining existing ones, it would be desirable to capture a symbol-lookup capability into a first-class abstraction.