This patch adds the new **executor-side resolver API** as suggested by
@lhames. It introduces a `DylibSymbolResolver` that helps resolve
symbols for each loaded dylib.
Previously, we returned a `DylibHandle` to the controller. Now, we wrap
the native handle inside `DylibSymbolResolver` and return a
`ResolverHandle` instead. This makes the code cleaner and separates the
symbol resolution logic from raw handle management.
This moves the MemoryAccess interface out of the ExecutorProcessControl
class and splits implementation classes InProcessMemoryManager and
SelfExecutorProcessControl out of ExecutorProcessControl.h and into
their own headers.
